I'd go with VC now because it's rarer, and you should probably wait and see if they'll re-release KZ2 with DLC included like they're doing with LBP.
I think Killzone 2 is the better game, however. If you aren't scared off by the class system and devote a decent bit of time to the online multiplayer, it becomes rather addicting. Of course, I'm somewhat biased in that I love objective-based shooters like this and Battlefield.







