For low pressure tires, like MTB, I prefer Schrader. For high pressure road tires, Presta all the way. The Presta's are a little less convenient to inflate, but the valve is designed to allow less pressure loss when disengaging the pump. Once the adaptor is on the Presta, I don't see much difference from a Schrader. As was said before, the pump makes more difference than the valve.
As for not being able to find a Presta valve tube at Walmart while on a long ride, I don't understand why there isn't a spare tube in their fanny pak. Of course the last time I needed the spare tube, it was for a group ride partner who doesn't believe in riding with a spare. Come to think of it, he still owes me a new tube.
