Presta valves are simpler, lighter, and require a smaller hole so your rim is stronger. They're also marginally easier to inflate to high pressure since you're not fighting the spring on the Schrader's valve core.
Schrader are more convenient for recreational cyclists because you can pump them up at gas stations without an adapter. They're also less prone to shifting and tearing when a tire is inflated to low pressure, so many mountain bikers prefer them.