I don't play any of these 2 games (BF3 & COD) but what i can say is the single player campaign playthrough (Youtube) of MW3 hooked and interested me more than BF3. I can't get past BF3 story after the early sniper scene, it just ridiculous for me. The game (BF3 or more accurately, the fans) proud itself for realism, but i think that sniper was little bit amateur; he exposed himself quite too much on that building. But that's not the reason i stopped watching, the real reason is i found the story a little bit uninteresting, even on the beginning.

p/s: For me, only ArmA got it right when it come to sniper (AI & human). Most of the time you didn't realize where they come from. But maybe that's due to lack of indicator on the game.