If it isnt on steam, it doesnt exist...BF3 is dead to me. So MW3 if i had to choose, although I kind of lost interest after COD2. Wont be buying either.
I played the BF3 beta's.. the servers seemed ALOT better and seem to have been fixed, much more realistic than COD as if you shoot someone they wont turn around and kill you... lay down whilst shooting and kill you or jump around a corner and kill you.
plus the maps are massive!! , although i'd get both think id spend more time on COD with a group of friends as it bores me without and BF3 when no one else is online as it will be more fun than cod alone.