I have both games. So i can say that both games are pretty decent. But i do think there are problems with each.
Call of Duty: Black Ops is pretty solid. But, for me, Modern Warfare 2 was just better. And that's how it worked when World at War came out. You can milk a lot of time from Black Ops, and personally, i like the new Wager game modes. I play those more than i do anything else in that game, thus i am a low rank =(
Battlefield Bad Company 2 is a really nice addition to the Battlefield family. But for me, it's just not as addicting as Call of Duty. So i don't get a lot of time on it. I even got the Vietnam DLC for it, but i haven't played it yet.
Overall, i think both games are very good shooters. And i enjoyed playing both games. But i didn't play Bad Company 2 much because i thought Call of Duty was more addicting. So, in my opinion, Black Ops is better. But that is just barely. Both games are pretty dang good.