There are pros and cons to each game. As I primarily play MW3, I would biasly say that I like MW3, but there were cool things in Black Ops that I wish they made more universal throughout all of the CoD games. I loved playing Zombies in Black Ops and was hoping that MW3 would contain the similar 4 player experience, just not zombies. Unfortunately that wasn't the case. It was fun with 2 players, but when you have to choose one friend to play it, while the others just go play Multiplayer.
I mean we really play these games for the social interaction with other people, whether that is to have a good time with friends, or to put "I think I am better that everyone" gamers in there place. Both actions generally end up being the case.
Dv