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.

I know that, but what i tried to highlight before was the sniper location at the beginning of the campaign was little bit off. He fired wildly at user controlled character when user reached top of the building, and exposed himself to be RPG-ed by user. I think the purpose of that scenario was to demonstrate destruction capability of frostbite engine.