I've always liked the SC's - best stealth games imo. Michael Ironside is in it, what more do you need? SCDA kinda had people going nuts with some of the story decisions regarding some favorites but that'll probably have no relevance to you. At those prices I highly recommend them.
I'll probably bite on AC at that price... Steam is so dangerous, with the prices and their sales it always makes me rather indiscriminate with my purchases.