Always use presta - on all my bikes. I used to use schraeder when I first got an MTB but when I met the SO I eventually changed to presta - can't remember what the justification was but all the people I ride with use them.

That doesn't mean that they are better than schraeder at all. Use what your are happiest with. I find presta easier to work with for inflating but I'm sure that is probably because that is what I use all the time. I have Mavic rims on my mtb (317s or 717s) they have schraeder sized valve holes but the presta work just fine.