Honestly, the Black Ops 2 multiplayer has some new scopes (ooooh x-ray scopes, innovation), some new guns (or more reasonably, new gun designs), same old game elements like stun grenades and turrets with new looks. Plus some new gadgets which look like they'll induce balancing issues.
Honestly, it's the same old COD, and the campaign looks like it was written, directed, and produced by Michael Bay. At least the first Black Ops took place in a time period which isn't often seen in video games. Now they're just jumping to the popular destination of the future, as they're scared of actually doing something new.
Get Assassin's Creed 3, it takes place in an open world, the combat system beats gunplay by a longshot, and it won't get old incredibly fast. Plus they packed entirely new features into AC3 like hunting, seasons, and a bunch of other stuff they haven't yet revealed. Also the campaign will be brilliant, and it will make Blops2 pale in comparison.
If you're already getting Halo 4, then just get AC3, because you don't need two shooters that take place in the future.