It depends; Crysis 2 has a background story, which kind of continues from Crysis, so if you're into storylines, Cysis is the way to go (which would mean you'd have to find about Crysis, or play it). There is no continuation of any storyline in Battlefield 3, but it's got a good single player campaign and awesome multiplayer, as it is already being reported.
Also make sure you have a PC good enough to allow you to play either of the two games, they need good resources.