MW2 is more fun than Black Ops, but Black Ops is more balanced than MW2. That being said, BO has a few things in it that are completely over-powered compared to others. For example, the perk Ghost Pro and the weapons Famas and AK74u. In MW2, all of the weapons are freaking overpowered so they balance themselves out, and the perk balance is better, though the One Man Army+Danger Close+Grenade launcher combo is game breaking. The killstreaks in MW2 are more fun than BO, and there is more variety in classes in MW2 than BO. The connection is also better in MW2, but BO has theater mode which adds a nice touch.
Personally, I would pick MW2 because you will have more fun on it than BO, and because MW3 will be similar to MW2 and MUCH different than BO.