They both have there stegnths and weaknesses Battlefield 3 does not have doplhin dive or zombies, but call of duty does not have "gravity" or the spotting feauture i think theyre both awesome but i like battlefield a slight bit more
I catagorize in close to long range fighting:
Close range: Ak-74u with rapid fire. Semtex, Concussion. Perks: Steady aim, Sleight of hand, Hacker if i sense there will be alot of claymores or marathon.
Longer range: Prefferably the famas with dual mag, Or ak-47 with dual mag. Semtex, Concussion. Perks: Flak jacket, Hardened, And tactical mask So i can see when people throw flashbangs!