Blops2 will sell more then MW3, the games are set up to one up each other.
MW3 = Quickscopers all over the place and its very easy to do, MW2 is harder in comparsion.
MW3 = Players dying in 2-3 bullets instead of MW2 3-4 or blops 5+, less room for skilled players and all connection based now, in blops you could escape from battles not anymore.
MW3 = Explosive damage being cut down to almost nothing. Semtex and tubes landing at feet and not killing the player(possible bug?)
MW3 = Horrible maps, worst in the series. Too small and too cramped, no room to even safe spawn for 1second and shoot down air support.
MW3 = Broken spawn system, its just as bad as MW2 for placing players next to where they died. kill a guy then turn the corner and he had respawned. Even sometimes in the same room(died to this last night).
MW3 = Commando is back. I was killed atleast 30times so far to commando knifes of around 10feet. Blops was around 5feet at the most after patches, most times you got no lunge though and just the swipe.
Blops2 will see these issues then fix them. Blops is also known for great maps like Firing Range, Summit, Nuketown, Launch. I only played Domination but those maps were among the best in the entire CoD series. More cold war or Vietnam era battles for Blops 2 and you have an instant winner.