I ride MTB in a coastal town where all roads are dirt roads with lots of uphill/downhill. Nothing extreme, I just ride to relax and enjoy the landscape. I have mechanical disc brakes and after a 2-4 hours ride session my hands hurt (too much braking). I have never ridden with hydraulic brakes, but it seems to me that they will be easy on my hands. Should I change to hydraulics?
I had some Shimano XT hydraulics on a mountain bike, and now have Avid BB7 mechanical disks on my touring bike. I don't know whether to attribute it to the lack of cable compression or some other mechanical advantage, but there's no question that the hydraulics feel much easier and controllable. It is also a more linear feel (or "better modulation" is a phrase I sometimes hear used) -- you get more braking right at the beginning of the lever pull because you don't have to first squish out all the flex in the cable and cable housing.
By contrast, my mechanical disc brakes, while very strong, have a bit of squish at the beginning, and don't subjectively feel as nice. I still choose not to run hydraulics because of the increased vulnerability (hydraulic line punctures) and difficulty of doing field repairs (particularly inappropriate for a touring bike). But the hydraulics, subjectively, definitely feel much stiffer and stronger. Since you're a bigger guy at 100 kilos, and braking quality sounds important to you, I don't think you'd be crazy to want to try switching over to hydraulics to see if they make a significant difference.
In order to brake you apply force on the brake lever that is then transferred to the braking surface (e.g. disc and pads). The advantage of hydraulics is the efficiency in which force is transferred (i.e. reduced system flex). Cable actuated disc brakes will have more compression in the cable housing, resulting in reduced efficiency and potentially tired hands from the extra force applied during extended braking.
That said, you can greatly improve cable disc brake efficiency by upgrading the cable housing to "compression less" housing (e.g. Avid Flak Jacket). This could be a much more cost effective option than switching to hydraulics. I did this on my commute bike and the performance was quite close to hydraulics, but did take some time to properly set up.
I'd recommend giving your current brake installation a good service before considering spending money on an expensive upgrade.
Hydros are great (I have them on my bike and enjoy riding with them), but a well-maintained braking system should be more than adequate in all reasonable riding conditions. Hydros would have advantages if you're into mountain riding as a sport, but for day-to-day use you probably won't notice much difference.
You say you've never changed your pads because they work. A pad can last a good while to be sure, but they do tend to become gradually less effective as they wear (more force for the same braking needed). You might want to look into changing the pads for fresh, good quality ones. That should give an noticeable increase in breaking power.
Similarly, the discs or rims that the pads press against can wear, though that's very unlikely to be a factor unless the brakes in question are very old. What is more likely to have an impact is dirt and other contaminates on the braking surface. Mud and dirt, lubricating oil from your bike, the remains of old brake pad material and what have you can build up on the surface. If you clean the surface thoroughly then you should notice further improvements in the stopping power of your brakes.
The cable in mechanical systems can stretch, fray, rust and so on. As it's a mechanical moving part it also needs to be properly lubricated. As the cable stretches with use you should adjust the brakes to counter. You should also apply some kind of lubricating grease or wax to the exposed portions of the metal cable with the brake in both the released and applied state. Finally, if there's any sign of rust or fraying in the cable, get it replaced immediately!
All this maintenance should make a very noticeable improvement in the stopping power of your brakes. More effective braking means less effort needs to be used when braking, and hopefully less strain being put through your hands.
You may also want to invest in a good pair of padded cycling gloves, they also offer a measure of protection from strain on your hands.
If none of the above helps enough then that's the point where you might want to start seriously looking at replacing your brakes. However, doing the above is likely to be effective in reducing the amount of effort you need to apply when braking, and it's a lot less expensive then replacing the brakes on your current bike.
I have or have had bikes with mechanical rim brakes, mechanical disks (shimano deore), hydraulic discs (also shimano deore) and recently hydraulic rim brakes (Magura hs33).
The experience I had says that:
So, I'd say the hydraulic systems are well worth the upgrade, but unless some facts are taken into consideration, sometimes the magic might not happen, specially if the braking pads are not doing their job. Also, a v-brake with new and well tuned cables/cable-routers, killer pads and decent levers can beat most fancy disc brakes around.
Hope this helps!