Seeing as i own both games, i will give u my very insight to which game is better.
MGS4-
Probally the last in the Saga of Snake. Story is great, lots of cutscenes (the game is basicly a interactive movie), Great Graphics, Great Weapons, Just an all around great Game.Metal Gear Online is not that great but the real reason u should buy MGS4 is for the story not online play
KZ2-
Stroy is short and easy compared to MGS4. Graphics are good, Controls are kinda sticky but you'll get use to it. First person shooter but not the same as cod4 or codwaw in any way. The online play is really easy to rank up in. Killzone2 is just an easy Game Online and Story mode wise
So if your looking for a Challanging game with a decent Multiplayer part go for MGS4
Lookig for a short easy story, better multiplayer though go for Killzone2