Neither really... MW3 if you have to play one of them.
Black Ops is still a buggy mess for most people - those that say its "fixed" are probably the ones who didn't notice how badly it ran in the first place - I've tried the game on about 30 different PCs and ALL of them have the stuttering/hitching problems - if you have vsync on it masks it with the penalty of increased input latency. I still have problems with it randomly losing profile stats or not able to join a game after random changes to my loadouts i.e. I can't equip any light machineguns on all but one of my profiles for some reason or if I do I can't join any server til I change them again for another weapon. While there are some good ideas in it like being able to hack enemy equipment to your side which is a nice counter for claymores overall its a bit weak - the weapons feel "lite" like airsoft weapons, sound design is a bit hit and miss and some of the killstreaks/rewards/perks are just silly too much empty headed style over substance.
Both Black Ops and the new DLC for MW3 have some pretty poor maps - generally quite nice looking but gameplay wise very poor but some of the original release maps for MW3 are quite playable. MW3 tho has a massive cheating problem - which is somewhat mitigated on BO where there are some decent dedicated servers ran by communities with an active player base and active admins that actually know the difference between someone thats good and someone cheating and boot them off.
TBH as much as I love COD - even MW3 - Activision have driven it into the ground, there are still annoying bugs like the mic issue, sound randomly inverting left<>right, inability to set the lobby system to NOT re-join the game you just left all of which should have been fixed years ago and they've killed off competitive/community support without the ability to host dedicated servers properly or make custom maps/modes easily.
I have a blast playing HC TDM in MW3 now and again and really enjoy that but overall the game has become stale.
