I'm more of a Battlefield person, so I'll be going with Battlefield 3 if I need a shooter.
CoD has always been a bit hit or miss with me. CoD 2 was great fun, Modern Warfare was a lot of fun, and the co-op single player in World at War was alright, although I didn't care for the multiplayer. Modern Warfare 2 and Black Ops just seemed like more of the same, so I didn't try them at all. I'm not too into the "game a year" thing, either. A lot of times it feels like we are just getting an update instead of a new game.
In the end, I'm sure they will both be fun, I'm just more into the big maps, tanks, helicopters, and Conquest game mode in Battlefield.