I like Black Ops much more. My FPS background started in Counter-Strike, so I'm used to having ~5 shots to kill and actually aiming for heads. MW2 was fun, but it was such a twitch shooter and came down to who was lucky enough to see the other first, since 2-3 shots and it was over. I love the fact you can turn around in Black Ops and win a gunfight if you're a better shot. In MW2 if you got caught from behind, you didn't have a chance to retalitate.
I'm also a huge fan of the toned down kill streaks in Black Ops, and that they don't stack. It's an FPS, I play it to shoot people, yeah the UAV and a killstreak that gives you 2-3 free kills is nice as a reward for those doing well. But in MW2 when you get more kills with air support than your gun, it's stupid. If I want to shoot people from a jet I'll go play Ace Combat or something. The nuke kill streak promoted so much camping and lame OMA tactics, I'm glad it's gone.
I did find MW2 fun, until the community ruined it with Danger Close/One Man Army noob tubes. And as xepic said, the sniping trick shots ruin it too.