I don't know, you can't really go wrong since they're both bad choices.
On one hand, you have Resistance 2. R:FoM wasn't a bad game and had a lot of great throwbacks to classic FPS games that made it worthwhile and even had that cool narrator. Insomniac, hating fun as they do, decided to take the good parts of R:FoM out and replace them with even more bland boring mechanics. If you liked R:FoM, this isn't the game for you.
On the other hand, you have Killzone 2, the biggest baddest in your face shooter ever. The atmosphere is *incredible* but then you actually start playing the game and it just isn't good. I could lie to you, but it's even less inspired gameplay wise than Resistance 2, frustrating at points, and has some of the worst boss fights ever created.
If you want a shooter, get CoD4 (of MW2 in a few weeks) or any one of the better shooters out there if you already haven't. If you must pick between those two, I would say Killzone 2 is the smallest loss.