As for me, I don't have much free time for gaming and just play occasionally.
A game like bf3 is more like a team player and require intense strategy and slow-paced which is the opposite of what we get in mw3.

I'd more prefer to play mw3 for the fast-paced, adrenaline pumping action and quick kill just to loosen up my stress from a long day of work.
This game is also more intuitive, easy to learn and I don't have problem with camper as they're more prone to get shot compared to a running player..

It's all bow down to personal choice..
Doesn't matter if you like bf3 or mw3, we're still united as a community of online gamer...