It depends what you're into. Right now, I'm really into MW3 survival mode and co-op missions. However, the online multiplayer is absolutely broken. It is almost as bad as modern warfare 2.
So, if you're looking for the complete package, go to black ops. In my opinion, Zombies is not as good as survival, but it has a much more intact multiplayer game that will not have you flinging controllers across the room.