MW3 had a better campaign but BF3 has better multiplayer.
This.
And most people who hate on MW3 haven't even given it a chance, or just compared its multiplayer to BF3. I personally think you can't compare the two, because they're nowhere near the same.
MW3 is an
arcade FPS, not intended to be completely realistic.
BF3 is a
war simulator, its intended to be as close to reality as possible.
Therefore, I think it is stupid to compare the two, as they are completely different in many aspects. The only thing I can connect to the two is that they have guns and are generally played in the first-person perspective.