It depends What you want out of the game.
Allstars is at it's best in a local multiplayer environment with 4 people in 1 room. Just playing match after match, and going for more unlocks, and then maybe jumping into some 2v2 or 1v1 for a much more personal experience. It also has lots of single player value through arcade/story mode, and all of the unlocks and characters trials and challenges. This also has online (mix of local and online), but I never see much appeal of online with these kinds of games, in my opinion.
Where as Blops 2 is much more focused on the online multiplayer. It's all about 4 player zombie co-op, or the traditional Cod multiplayer (with some big changes to the formula this time). The single player isn't the focus, and neither is local multiplayer so much, although it does support 4 player split screen with bots, or 2 player splitscreen online.
Both games are very multiplayer focus experiences with lots to do, and lots to unlock. So from what I can see, would you prefer to play online with some friends, or just kick it back with from couch combat?
But who's to say you can't get both? Get one this month, and pick up the other the next month (or the one after that).
Personally I'd go for all-stars, but that's because I want to play it a TON with my roommates, and friends.