I don't own either game, (or a PS3) but I have played through most of KZ2 and as of last night, all of KZ3 at a friend's place.
Though the critics mostly disagree, I actually found Killzone 3 to be the better game by quite a large margin, largely due to 3 factors.
Aiming
I get what Guerilla were trying to achieve when they programmed Killzone 2's aiming. It's meant to be weighty and loose, like you're a real person holding a heavy piece of metal. Unfortunately, it simply doesn't work in my opinion. A trained and capable soldier (which Sev is portrayed as being) should be able to handle a standard machine gun with more finesse. At time it feels like he's drunk, swaying too far with unnecessary inertia when you try to turn.
Killzone 3 has no such problem. It strikes an almost perfect balance between feeling realistically weighty yet satisfyingly precise. What's more, it adds pointer control. Unfortunately my friend does not own a Playstation Move so I didn't get to try out that option, but if it's done as well as most quality Wii FPS games, then that's another major advantage over KZ2.
Settings
In my view, Killzone 2 suffers from what I like to call Conduit syndrome; strong graphics technology wasted on monotonous grey/brown environments. There's only so many colourless industrial areas I can fight through before it starts to get a bit tedious.
Killzone 3 once again corrects it's predessor's shortfall. Not only do we get an arctic environment, but we also get an alien jungle which is one of the most beautifully designed game environments I've ever seen, worthy to stand alongside the likes of Rapture from Bioshock, Light Aether from Metroid Prime 2 or the various worlds of Metroid Prime 3. The coral-like vegetation, eerie colour scheme, and attention to detail were just breathtaking.
Gameplay variety
KZ2 had a few sequences that broke up the man-to-man combat, like the tank and turret setpieces, but these were few and far between. The lack of variety in the enemies was also a bit of a drag.
Yet again, KZ3 to the rescue. There were significantly more vehicle sequences, including an awesome section where you use a jetpack to assault a weapons platform, which kept things from getting too samey. These jetpacks were also used by enemies, which in conjunction with they creepily animalistic Helghast robots, kamikaze spider droids, and other mechanical baddies offered a wider range of foes to gun down. (Even more variety would be nice, but it's still an improvement) There was also some stealth mixed in to spice things up.
In KZ3 I never found myself doing the same thing for long enough that it outstayed it's welcome, while in KZ2 I often did.
Killzone 2 isn't a bad game, but honestly, it didn't even come close to living up to its reputation in my opinion. Killzone 3, on the other hand, I found to be one of the finest console FPS games of this generation, at least that I've played.
What say you guys/gals?
