I would most definitly have to say Modern Warfare 3 because the game is much funner and just all around better. When Black Ops came out Many gamers including myself were very dissapointed with the gameplay, especially online. It was just not really like COD. Zombies sure is fun, but the rest of the game is just kinda boring. On MW3, they have an all new Survival mode which is the just like Firfight on Halo:ODST,Halo:Reach, and Gears of War. Countless waves of enemies come at you and it doesn't end until your life does. In the game of course, lol. The online play is great and 2 players can play on the same console together online as well as in spil screen,and survival. Black Ops is old news and I suggest you join in on the most online played game (on the Xbox 360 at least) in the world, Modern Warfare 3. It's worth the money. But so is Black Ops, if you buy a used copy for $20.
I would suggest Black Ops over MW3. If your really in to call of duty you wanna get MW3 because it's newer. I think that Black Ops is an overall better game