I'm planning to buy a bike - mostly using it in the city on a daily basis - sometimes long roads - But I want a cool bike and I think mountain bikes look very cool.
I would like to know how mountain bikes do on normal roads, especially in the city? Is it more tiring than a regular bike, or what are my other options for city commuting with the occasional touring ride?
For the past few years, I have commuted on a road bike for most of the year and on a mountain bike for late fall through early spring. My commute also involves both classic road-riding and more urban-style riding.
What I've Found
I've found that I prefer the road bike by a significant degree. Living in a hilly area, it is always a pleasure to drop the excess weight of the shock system, the efficiency loss due to bouncing and flexing, and the wide tired of my mountain bike come spring.
I can finally take my backpack off and put on panniers, which keeps my back from getting as sweaty and is generally more comfortable. My fenders work better to keep the mud off me. It is an altogether more enjoyable experience.
If I could ride my road bike in the winter, I would. Hard to do in the snow, unfortunately, especially when it comes to the turning bit.
What I Lose
However, when I switch to the road bike, I feel I lose some peace of mind. On my road bike, whenever I see a patch of sand at a corner I've got to take, whenever I ride in the rain over fallen leaves, I worry that I will lose traction and fall.
Furthermore, I've been run off the road before and gotten close to it several times and I feel much safer and more comfortable on my mountain bike knowing I can ditch the road and jump into the rough easily if necessary.
On my mountain bike, I don't fear to hop up or down the curb to, for example, avoid a runner in traffic. I can take shortcuts over uneven terrain, hop a few stairs, and be on my way.
Conclusion
In conclusion, I recommend finding a bike that fits your specific needs. If you can only have one bike, find the bike that allows you to do everything. If you live somewhere that gets significant snow, if you live in a place with terrible roads and worse drivers, you may need that mountain bike. If you live in a city with as many shortcuts and stairs as potholes, you'll love the "Go Anywhere" ability. If you live in a very hilly environment, you'll love the climbing gear on your MB. If you have any interest in actually mountain biking, keep in mind that a mountain bike can do both while a road bike has to stay out of the trails.
If you live in a more suburban area, or somewhere as flat as Kansas or The Netherlands, or if you live somewhere with nice weather year round, a road bike is the way to go. The ride quality will be higher; you will feel light as a feather, fast as a Ferrari, and much more efficient than on your MB.
TL;DR:
If the part of mountain bikes that you think looks cool is the rear suspension, I very much recommend you buy an uncool bike. Rear suspension is very heavy, requires maintenance, means you can't fit good mudguards, stops you using panniers and means that, while you're riding, part of your energy is going into flexing the bike around instead of moving you forward. It's also completely unnecessary unless you're riding down mountain tracks at high speed, which you say you won't be.
I would suggest a "hybrid" (also known as a "fitness bike" or "commuter bike") – they have a more-or-less mountain-bike-shaped frame and might have front suspension (which is a comfort-versus-weight trade-off that you'll have to consider). They have narrower, smoother tyres than mountain bikes, and these are better suited to riding on paved surfaces but they'll still be OK for light off-road use such as gravel tracks.
Generally mountain bikes offer certain advantages and disadvantages over, say, road bikes for commuting.
Advantages: The seating position is comfortable, and ridig will also be very comfortable due to suspension and the wider tires. Also, in bad weather, mud and snow you probably have better traction with MB tires. Also the wide handlebars make nimble riding very easy.
Disadvantages: The wider tires, higher weight and less aero positioning of the rider mean that you generally have to invest more power to go the same speed.
If a bike, that looks 'cool' to you means that you'll ride it more, then go for it. But it might not be the best possible buy. There's also bikes specifically made for commuting which focus on being very reliable and still comfortable (Canyon Urban and Commuter series come to mind).
To answer your question completely, we'd need to know how long your commute is, what your budget is and if you consider also riding trails with your bike.
I'm riding a hardtail 26" XC bike for commuting, never had a road bike, but tried once, so that's my experience:
Pros:
Cons:
One more to "pros", but very specific to where I live (Saint-Petersbourg, Russia) - there are lots of XC races, much more than road ones. So if you'd like to race one day - mtb may be better
upd
I've changed my old ammo fork to a rigid one, installed fenders, rack and bar ends.
Now this bike can either carry some goods or kids seat while allowing to grab some strava crowns - so yes, mtb can be good for urban riding, but with some modifications (and you still going to have relatively low top speed)
Especially if you consider commuting I would add at least one more option than the two you mention.
Road bike are intended for a fast rides on a rather flat surface and I would say they aim more for a race riding so that's probably not what you want to have for a daily commuting. The position is not comfortable, it is optimised for limiting your drag coefficient and getting most power out of your muscles. Such bike has a seat positioned higher than a handle bar, often with ram handles to enable even lower position (sometimes handle suspension in the middle). They have thin tyres and in general are very light. They often aren't fitted with fenders or even lights to save on weight.
These bikes are optimised for a difficult mountain paths where there are heavily changing conditions, steep slopes, mud etc. Yet the biker does not really care for that (i.e. getting dirt). They'll have a bit more comfortable position than a road bike, with seat just below the handlebar. They will have wide tyres with deep tread, plenty of gears, often additional handles sticking up perpendicular to the main handle to enable shifting position when needed. Usually fitted with some sort of shock absorbers.
If you've ever been in Netherlands, you've seen plenty of those for sure. These bikes are designed to work in a typical city conditions. Dutch are freaks about bikes (I've been told an average Dutch has at least 4) and they really know what they are doing. Typically city bike has a very upright position. It is far more comfortable than anything you can get in either road bike or MTB. Tyres are quite wide but with tread not so deep as in MTB. Handlebar is much higher than the seat. They often have a single gearbox in an enclosed case to reduce maintenance with 3, sometimes 5-8 gears. Also usually fitted with luggage space (sometimes also in the front), fenders, lights, protection so that your dress don't get into the wheel, chain cover and everything to ensure that a gentleman in his suit or a lady her dress don't get dirty and can get into the office or whatever other business they have. These bikes are really heavy (20 kg is a typical weight) and intended for biking on a flat surfaces but handles small terrain obstacles too. Sometimes are equipped with shock absorbers, usually have very comfortable saddles. This is all about your comfort, not speed.
If you don't plan too much off-road I would really consider this option.
All types listed above have some cool bikes. You may need to look for it a bit more. I own a Dutch-style city bike that I really believe is cool (Batavus Mambo) but I had some MTBs and road bikes in the past as well.
What I would consider is making sure your bike is comfortable for you. Otherwise you'll just stop using it way too soon.
in addition to the other replies I'd like to emphasize:
the mountainbike makes potholes less of an issue. Partly due to the actual suspension, but also from the suspension provided through the tires, and by the larger inertia of the tires. So while no bike will instantly disassemble under you when going through a pothole, you probably need to be react to them on road bikes. While on a mountainbike you're way further away from losing control even if a pothole hits you completely unexpected.
tram tracks: If you ride in a town with trams, wider tires have the advantage that they don't slide into tram tracks as easily as narrow tires. Even if your mountainbike tires are not wider than the tram track, you usually need to hit them at a very steep angle to get into trouble. Road bikes are less forgiving.
So these points are pro mountainbike: You need to worry less about the ground ahead of you and can spend more attention on traffic. BUT this should only be a comfort argument. If the road and traffic are so bad that they are a real issue on a road bike, then maybe riding there isn't such a good idea on a mountainbike either...
Con:
EDIT: I am talking about energy, not force. I assume any bike with gears should allow you to pedal in city traffic at a cadence and pedal force/torque of your choice with the achieved acceleration as remaining degree of freedom. At a given cruising velocity, that energy - that dissipates into heat once per stop (independent of trip length) - will be higher for a mountainbike than for a road bike, due to the higher mass and higher moment of inertia/angular mass/rotational inertia. (And when considering lower cruising velocities for mountain bikes than for road bikes, that may or may not still hold, my feeling is it still does for my bikes and cruising velocities but I'm not going to buy a power meter to run tests on it)
For similar riding, I actually prefer the mountain bike frame over a road bike. I like the stability of the wider tires, and ability to cut across grass or gravel areas without a second thought. However, I look for a few specific characteristics:
The bike should only have front suspension (no rear, too much weight for not enough benefit), and maybe a seat spring.
Look for tires with a smoother tread pattern. This will make for a much smoother and easier ride, while still providing extra traction and stability for places where the road might not be so nice.
More gears aren't necessarily better. The drive on a mountain bike is likely to have some extra low gear ratios for easier work in tough terrain that just aren't as necessary for rides that are mainly on the road. Riding uphill on asphalt is just not the same punishment as riding up an actual mountain, and a bike with fewer gears is likely to result in easier, cleaner shifts. (Of course, there's also a "you get what you pay for" factor here).
And again, these are my preferences.
You can ride a mountain bike on the road (you might not be as efficient or able to keep up with people on road bikes though), but you really can't ride a road bike on a trail.
You asked if it is tiring, no, not really, so long as the seat is properly adjusted and you use the gears properly it will be fine.
There are things in between though, for example my first higher quality bike (that came from a bike shop as opposed to somewhere like Walmart) was a hybrid. Its tires were still fairly narrow but had ridges, but not as narrow as a road bike's. It had maybe an inch and a half of front suspension and no rear suspension. I was able to ride it on trails, but a real mountain bike with rear suspension was much more enjoyable once I got one. That hybrid was great though, I used it to commute from my college dorms to campus (in a busy city with potholes) so the suspension made it a lot smoother.
Lots of answers already, but i'll try to give my point of view.
I'm a Dutchie and regularly use both bikes, but each for different purposes. They each have their pro's and cons. MTB is a mid-end hardtail and city bike has a simple three-speed geared hub with an enclosed chain guard.
Pros
Cons
Pros
Cons
In short: a mountainbike is more fun to ride, but a city bike is easier to live with and way more practical for daily use.
Another aspect that hasn't been mentioned is your tires. Mountain bikes typeically have knobby tires and road bikes have smooth tires. I have a mountain bike, and when I wanted to start training for a week long ride, I had no real option but to ride on roads (because we don't have 60mile long trails where I live). Road bikes run with a lot higher pressure in their tires which limits the amount of friction you have with the road. If you increase the pressure in the mountain bike tires ( knobbys) to their max, your ride will have an extra vibration because of the tread on the hard road, and will take a bit more effort.
One way to eliminate this issue and still not have to invest in an additional bike ( and figure out where to store that second bike) is to buy some slick tires for the wheels of your mountain bike. These will typically hold more pressure than your average mountain bike tires so you will get a smoother ride with less effort.
If you do a lot of both types of riding you can invest in a second wheel set and have both types of tires ready to go at a moments notice.
What others have said about frame types is certainly true, but this is a way to have a little of both if you are limited to one bike! Have fun riding!
Others have given pretty solid answers but I would like to add that there is another kind of bike made for everyday commute apart from city bikes.
I believe they're somewhat "exclusive" to France in retail (but you can build one yourself very easily), we call them "vélo tout-chemin" which means "all-road bike" in contrast to mountain bike which we call "vélo tout-terrain" (all-terrain bike).
The concept is the following : a simple mountain-bike frame with only front suspensions and tires with thread depth variations : the middle is flat like road-bike tires and the sides are thick and ridged like mountain-bike tires), but I don't know how hard or easy it is to find that outside of France.
So with such a bike you can put less effort in riding on road than with a mountain bike while keeping a good look and not fearing potholes or rain. You also keep the ability to do light off-roading (like forest tracks for example).
If there are no such bikes in retail where you live, you can achieve the same result by buying a simple mountain bike and a second set of quick-change wheels with city-bike tires or simply equipping bicycle-touring tires if you want a single set of wheels.
There is only one real answer: try out the bikes you want to ride.
I happen to ride either a BMX or a rigid ~10kg MTB with 67 HA and 2.35 tyres (the front is a soft compound, 45 watts in rolling tests and I run it at 27PSI, but this is worth it for how I like to ride), in the city. I do this because I enjoy it.
So yes, a real mountain bike, with the fork change to rigid is absolutely awesome to ride in the city, if you're like me, and if the bike is good.
But you're not me and you don't have my bikes, so all my answers and those from anyone else will never help you except by wild coincidence.
Buy the bike that makes your forwards progress as lightweight as possible: a road bike, not a mountain bike.
I long time ago thought that buying a "hybrid" bike was a good idea. Best of both worlds, that was my idea.
Soon later, I switched to 28mm slick tires. Rolling resistance problem solved!
A bit later than that, I switched the handlebar to a drop bar, with shifting using bar end shifters and with braking using those drop bar brake levers made for V brakes (which I eternally hated). Air resistance problem solved!
What in the end I was annoyed about was the suspension fork. Couldn't change that, so I built myself a genuine road bike.
I have ridden with the hybrid to road bike conversion and with the true road bike on many terrains. 28mm slick tires work just fine on most acceptable quality forest paths. MTB may be better in the mud or in soft wet sand, but how often do you encounter those situations? (Besides, ever heard of cyclocross? They use what are essentially slightly modified road bikes on harsh terrains.)
You said your riding is "urban", on long routes using normal roads. That's where a road bike really shines. If your routes include forest paths, you may want to switch those stupid 23mm tires to 28mm ones. You may want to do that anyway, as after fixing a flat 28mm tires require less pressure than 23mm tires. Also, 28mm buys you slightly more ground clearance, which is good.
Oh, and road bikes are really cool. If your idea of cool includes riding on harsh terrains (although you didn't say you are planning to do that), you can always buy a cyclocross bike which is essentially a slightly modified road bike.
