Depends, if you prefer the multiplayer and co-operative side of things go for Black ops 2, but if you want a brilliant story mixed with fantastic gameplay, AC3 would be your best choice.
I don't really enjoy COD much myself, it's getting way too repetitive but people still buy it because all of their friends do. The only thing I really enjoy about BO2 is zombies, that alone is worth buying the game for. I think I'll wait for the hype to die down and get it later, right now I need to get Halo 4 and AC3.