I think it can be put roughly into the two categories:
Gears2: Characters look more detailled up close, some really really awesome lighting.
Killzone2: Much more small details going all the way into the screen. (Gears2 has also buildings in the background but it always looks a bit faked, in K2 you see small details as far as the resolution will allow), better particle effects like smoke, much more things going on, far better character animations.
Gears2 looks like a render movie with a focus on characters and enemies (the predator style enemy looks amazing). Killzone2 succeeds in all videos in creating a war-scene with explosions, smoke, enemies all over the map.
Both are technically amazing and both have their strengths but I think which is better is depending on your taste.







