They are two completely different games, if you like team play bf3 is more of a team player game and more realistic, vehicles. MW3 is more of an action game.
Battlefield is only good because I love to destroy buildings and prevent some campers around me. COD is going to nerf my noobtube because HATERS GOTTA HATE. I still noobtube in both games regardless.
You definitely can not compare these games. They are completely different styles of FPS's, battlefield seeming to be the more realistic and MW3 seeming to be slightly more arcade like. I personally like Battlefield more but the Beta doesnt have much going for it.