I honestly don't own either, but from what I played of each MW3 is a $60 MW2 map pack, and BF3 is actually a fresh game.
Battlefield (as a series) is a lot deeper of a game than Call of Duty ever was, so at least from the perspective of long term rewarding gameplay there's really no question. But of course, CoD is still good when you feel like completely zoning out to a shooter when you don't feel like having to think. Much less rewarding, but is still ideal sometimes.
So Battlefield 3 I say.