I'll choose not to choose.

Picking both up

Playing both now.

SP: BF3's campaign story feels a bit ho-hum. Lost interest halfway. MW3 is a bit more action packed and engaging, but still is the same formula since MW1. Wish they're both more open ended like Crisis instead of me needing to "Follow" somebody all over the map.

MP: Playing as a pubber.
BF3 is much more engaging when it comes to teamplay. Downside is that in most games, one team will usually get steamrolled and spawn camped. Hard to find even games.

MW3 you'll get more closely contested games, faster paced. But plenty of lonewolf running around with akimbo guns blazing. Teamwork? Not much. Not much chatting too.

MW3 still looks similar to previous MW games, but for some reason, does not runs as smooth as BF3.