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Ok. The choices are:

Mass Effect 2. Just got released on the ps3, got GoTY awards (360/PC) on several sites,
A shooter/Rpg hybrid and a game which all choices you make has a
consequence. Whether good or bad.
Replay Value is pretty high as you can make your second playthrough
very different if you make different choice/change your path.

Killzone 3. Im a MAJOR fan of Killzone. But it has always been weak on
story.. But Killzone 3 promises (also trailer shows) the Story will be
great! Killzone has great graphics and also has intense gameplay.
(Killzone 2 in my opinion, still has the best graphics on ps3. Uncharted 2.. Im sorry.)

While I am a major fan of Killzone,
I am wanting a game that i can replay all over again, without getting bored.

I've heard Mass Effect 2 does that very well.
But Killzone 3 will obviously have the intense shooting action I'm
craving for.
Killzone 3 will have co-op. (Couch Co-op.)
Mass Effect 2 will have an RPG element to it.
(btw, Makers of SWKOTOR1/2 have made mass effect 2. Bioware.)

Please help me choose. :]

Well I've played ME2 on the 360, and it's a brilliant follow-up to the original. The graphics are great, the story is really good and the action is, well if you liked the first one you'll love this one, personally I wasn't a fan in ME but it has improved. I've never played any of the Killzone games so I can't pass judgement on them, but obviously if you want multiplayer shoot-em-up go for Killzone, if you want an interactive story which changes each time you play the game, go for ME2.