What kind of valve is the best one, a Presta valve (6mm) or a Schrader valve (8mm). Which has the best reliability, which one is easiest to fix a flat on?
Presta's main benefit is more about how the valve functions, in regards to getting pumped up to higher pressures. Because the stem of the valve needs pressure in the pump head to get high enough before it pushes the valve stem in, and start flowing air into the tire, when you try to get to 120 to 160 lbs of pressure it works better than a Schrader valve where the valve stem is depressed the whole time, back pressure can leak out.
Otherwise I would say tomato - tomahto (potato - potaato?)
- smaller hole in the rim is good (presta +)
- schrader has piece that inserts into the valve (screws in) with a spring mechanism to seal it - these parts sometimes fail (schrader -)
- presta uses tire pressure to seal it (presta +)
- rocks can lodge in uncapped schrader and cause leaks (schrader -)
- presta does not need a valve cap to keep rocks out (presta +)
- presta has a locknut to prevent valve movement (presta +)
- You can prep a presta valve tire for installation without a pump - using your mouth (presta +)
Presta is better but is insignificant for most people. Use the tube that fits your rim.
My conclusion after many years of using not two, but three types of valves is that the best is the one that results most practical for you, acording to type of riding, type of pumping methods available and of course the type of bike/tire/rims you are using. Neither valve type is absolutely better than other, but one of them may result better for your particular circumstances.
For example when I was younger, the modern mountain bike was not yet popular in my country, so the most available tube had dunlop valves, also, the classical stick pump was available in the only bike store, in many hardware stores and even some supermarkets and department stores that sold sporting equipment. Back in that time, it was crazy to have a schrader valve, because you could only buy cheap and bad quality foot pumps, and the gas stations here where too careless to mantain their pumps in working order.
Nowadays, I have schraders in my DH bike, presta in my XC bikes and dunlops in my wife's XC bike. Besides that, I have many presta-to-schrader adaptors in my toolbox, patch kit, in my car, bacpack, etc. So I use a schrader pump head most of the time. This gives me huge flexibility, I can pump with whatever I have in hand! That is, In the car I have a small "compressor", an electrical pump that runs on 12v from the car's battery. At home I use a cheap but decent floor pump that has schrader head from a hardware store. Here the advantage is that it cost a fifth of a "proper biker's floor pump". And my Mini-stick pumps are set to schrader mode. Yes, I preffer to screw in the adaptor than to reverse the inners of the pump head.
I still have some dunlop tubes because here you can buy up to four or five dunlop tubes for the price of one schrader or presta. I use schraders on my DH bike because of the high volume. Schrader provides a wider air passage, so they are less restrictive and that allows for faster pumping with a hand of floor pump. Obvioulsy a DH tire does not require high pressure. And the prestas are used because higher quality tubes or certain specific sizes are only available in presta/shcrader.
After about 20 years of riding BMX (as a kid), Road, XC, DH, and even BSOs, i have never noticed a significative difference that forces me to choose one over another, and I find equal difficulty inflating them or repairing flats in any tube type.
The only possible advantage I see en prestas is that when you need to deflate them, you unscrew the valve closure and press it with yout finger, you don't need a thin object like with schraders. (such an object may already be in your emergency tool set, for example a 3mm hex from a multitool does the trick). The other advantage may be the locking nut that avoids the valve receeding into the rim when pressing the pump head against it.
Presta may be mandatory for narrow rims, like the ones in road bikes. Dunlop and Schrader both require the same diameter for the rim hole.
Of the three valve types, only dunlop actually requires air pressure to both open the valve while pumping and to seal it, and in practice it is the only one that closes itself while the pump is being filled with air. The typical stick pump for dunlop valves would not have a check valve on the output.
Both, Schrader and Presta specific pump heads press the inner pin or the closing nut respectively to open the valve. (I'm not saying prestas cannot be opened by air pressure from the pump, but most pump designs press on the closing nut). For that reason, modern pumps do have a check valve or an equivalent mechanism.
After that being said, whether it is easier to pump to a certain pressure depends more on the pump design, including the pump head. Some manufacturers simply have a better presta design than a shcrader design, but the opposite may be true for another make/model of pump.
That is, given that you have a good pump, it is equally as hard/easy to inflate a tube, and you will always find people that swears by othe or the other type, but the choice is yours. If you actually have the option, choose the one that is compatible with the best affordable pump that you have or can acquire.
Many riders and mechanics recommend discarding the lock nut on the Presta valve because it causes more problems than it cures. I got rid of the lock nuts on the 4 bikes I have that use the Presta valve and have experienced no problems after doing so.
As to valves fitting rims...you can drill a 3/8 inch hole in a Presta valve rim and use Schrader vale tubes in it. You just have to make sure you find a tube that matches the recommendations for tire pressure of the tire. Also make sure you "clean up" the hole so that no rough edges or burrs are present.
A schrader valve require air pressure to open, A presta does not.
The biggest difference will seen when airing up a tire past 100+lbs, A shcrader will take more pumps to get from 100 to 110, because at that high of a pressure, more air is required to open the valve, whereas on a presta all the air goes into airing up the tire.