Definitely hold 'em.
There are tons and tons of resources to help you learn hold 'em - books, training sites, forums like this, the list goes on. There's also endless amounts of action at pretty much all stakes, so you've got pretty much unlimited opportunities to practice.
Stud, by comparison, has much more limited resources for learning and it has less action, so there's less chance to practice too. Plus there's less opportunity to really use your skills once you do develop them.