The 2 games are different in game play. BF3 is more open warefare, where MW3 is more urban warfare.
They both have their plus and minus points.
Just yesterday my best bud converted another friend, MW3 only player, to consider BF3, just with the BF3 Gamescom video.
From a survey most players will buy both games, thus just smiles from both companies.
To me, this is starting to look like the ongoing console war. I'll say what I always say: Each game has its pros and cons and in the end, it'll come down to personal choice. Personally, I intend to get both
I thoroughly enjoyed the first MW, COD 4, but since then it has kinda felt like more of the same. Whereas the multiplayer on Battlefield always keeps me coming back. But I'll definitely be getting both!