Is it better to use presta or shrader valve for tubeless conversion on a mtbike, 29" 2.20 tire, and why?
The debate of Presta or Schrader is mostly religious and often based on poorly misunderstood facts or historical differences.
Rim Width. In the 21st century, the problem is less about the strength of the rim and more to do with fitting the tire beads and valve into a narrow rim. If the rim is wide enough to accept a Schrader and mount the tire, its strong enough. As we are discussing MTB's – the last time I ran a rim too narrow for Schrader on a MTB was 1993. In the last decade, MTB rim widths are getting wider so it makes no difference what valve you run.
Weight. Presta are marginally lighter; only you know why you care so much.
Length. Although long Schrader valves are available, if you have deep rims you are more likely to find Presta in long valves.
Pressure. It's commonly stated that Presta is better at pressure – Schrader are used to contain pressures of over 600bar. They are used in MTB shocks (up to 250 PSI ) and your car's air conditioning unit (200PSI, expected no gas loss for a decade), so I think that myth is busted.
Inflation. Presta had the edge way back before the days of plastic heads and computer engineering. In 2017, if your pump does not have a head that successfully inflates Schrader every time, buy a pump worth more than $1.
Robustness. From personal experience and anecdotally from internet discussions, Presta is prone to having the end broken off by some combination of one or more of clumsy novice and cheap pump. I have not done in the last couple of decades... Schrader really do require the caps to keep rubbish out of the core (although plenty don't and seem to last OK).
Reliability. Schrader are used in every car tire in the world. Presta are probably at least as reliable, nuf said.
Valve Length. Although tubes with long Schrader valves are now available, it's more common for rims requiring long valves to be Presta.
So why use one or other? I mostly run Presta because many people still think Schrader is a mark of a BSO, and although I ride a 10 year bike, I would not be seen dead on a BSO.
For tubeless though, I think the balance swings to Schrader – bigger hole for putting the slime into the tire and for initial fast inflation to seat the bead.
This is a good appraisal, but I would add one thing, which is especially for the traveller. If you’re on a long journey, say, multi-month, maybe even multi-country, there is distinct possibility that you might lose, damage or break your pump. Travelling lightly, your unlikely to have a spare one. Therefore, I would recommend Schrader valves because, pretty much anywhere in the world you can find a pump that can inflate your tyres. This is, of course, because all cars, motorbikes, trucks and buses use Schrader and often have a pump, too. That also means that practically every gas station and auto workshop has a compressed air pump with a nozzle for Schraders (though this requires extreme care to use, and is not suitable for high pressure tyres. Mountain bikes should be OK.). So, for expeditions, I’d say use Schraders on a mountain bike. Or carry an adaptor to be able to use car pumps.