It depends... Do you want only card related stuff? Or do you want magic with a lot of things. If you're a serious card worker and would like a lot of sleights to work on, get Card College. If you want magic with lots of different things including cards, coins, paper bills, cans, matchsticks, etc. then get Art Of Astonishment. Personally, I'd recommend getting AoA unless you're a card sleight junkie (there's nothing wrong with that).
PS. AoA will require you to know a few basic sleights so consider that as well. CC will teach you from basic to advanced card sleights.