I'm not into either one of these, but if i'd have to choose...
-> Battlefield
Why? I haven't even ever played battlefield. But i used to play alot of CoD. I don't like CoD because it seems there is surely always a weapon that will obliterate anything else. (MP40-Waw, One man army noobtubes-MW2, FAMAS-Black Ops, MP7- MW3). And the worst thing?
THEY NERFED RIOT SHIELDS!
MW2= unbreakable beasts that absorb bullets like it's no big deal, and actually protected you.
MW3= shields break in 2 shots, and you can just spray at it, and you'll Always get a kill anyhow.
And i also dislike the amount of ragey kids in CoD games, i have nothing against them playing it, but squeaky voices get annoying after a week or so.
I go for the game i have yet to experience, as i've heard good stuff about it.
BTW: best FPS : Gotham City imposters, hands down.