MW3 will win the multiplayer battle in the end. In 1 year from now, MW3 will be played much much more online, while BF3 will only be played by 10% of it's buyers by then.
CoD just have way more experience on making a solid multiplayer (as in one not dying after 6 months).
I would pick CoD MW3 above BF 3 any time.
Bullshit, in a year people will be playing Modern Warfare 4.
Okay, and don't you think if people again buy that game (which probably won't be Modern Warfare 4, will be a title like BLOPS and WaW), they were really happy with the previous game? And I don't know the exact numbers, but it would surprise me if BLOPS still isn't more popular than BF 3 online. The game is a year old but I'm pretty sure people still like it and play it, more than people who bought the brand new BF 3.