Not really the same type of game to be honest. Sure they're both FPS but they cater for different types of gameplay. For me COD caters more to the "Let's have a few quick 5 min rounds" people and BF more the "Let's go to war for 30 min at a time" type of people.

These days I'm falling more into the COD side since it has less issues at the beginning (with the obvious exception of BLOPS ), but I'll give BF3 a go when it comes out.