There has been a lot of talk online about these two, particularly in whom has the superior graphics. I have been playing through them since the beginning of the month so I feel I should do a tandem review on them both (note: played the PS3 version of Crysis 2)
While they are both FPS', they are very different. In a sentence, Crysis 2 is Cloverfield starring the Predator and Killzone 3 is the Saving Captain Narville.
First, the highly anticipated exclusive Killzone 3. 3 starts moments after 2 and continues the story of Sev and Rico, while focusing heavily in the fallout of Visari's death, and what happens next in the Helghan government. As with all the Killzone games the plot is solid if predictable, but it's the setting that sells the game. You really feel like the ISA/Helghan conflict is a tangible war. Seemingly inspired by Iraq and WW2, Killzone 3 is grounded in a realistic war; no silly alien zealots or far fetched communists here. Crysis 2 on the other hand discards the intriguing 'less is more' plot of Crysis, focusing on a far more generic PMC vs Alien conflict. This could be thanks to a new writer, who fails to address the events of Crysis and pens a poor script. I would expect better from a pro.
The gameplay of both is like chalk and cheese. Killzone 3 retains a largely similar style to it's predecessor; Cover is essential and and the Helghnan are fierce opponents with cunning AI, but 3 makes many improvements. The controls and cover mechanics have been tightened up, and your squad AI is vastly improved. Killzone 3 is very linear and scripted, and the campaign is a fair length at around 7 hours. It will appeal more to Call of Duty fans, even if they'll have a bit of a learning curve. Crysis 2 is all about the Nanosuit; the auto asphyxiation aid doubles as a predator simulator, and sulking around the battlefield is very gratifying. However the suit does become something of a crutch; it's all to easy to stay cloaked, sneak up on an enemy, line up your shot, uncloak, shoot, recloak and repeat. But there isn't anything really like Crysis 2 out there at the moment and it's a great buy if your tired of 'modern' FPS', and a 10 hour campaign with New Game+ makes Crysis 2 one of the best single player shooters since Bioshock.
The graphics are the real hooks to both games. In short; there both fantastic, but the slight edge has to go to Killzone. The models seem to be of a higher resolution, the frame rates more consistent and there's a stronger art style (Crysis' Ceph look like a cross between the Silver Surfer and Krang from TMNT). But the fact that Crysis 2, a multiplat, comes so close to a PS3 exclusive is very promising, and it puts recent efforts from other third party developers to shame. God only knows how good Codename: Kingdoms will look. There are bugs in both games though. There are a plethora of minor, annoying bugs in Crysis 2, but Killzone 3 broke for several days while I was attempting to infiltrate Stahl Arms, causing cutscenes to freeze and my console to lock up. Thankfully Guerilla patched the game quickly, and seem to be correcting there errors.
Before the PSN outage I got some time with the multiplayers of both games, and yet again there very different. Crysis 2's multiplayer is essentially MW2 with a Nanosuit in New York. The predator feel is lost and the string of unlocks is getting too tired to continue to act as a carrot on a stick. It was handled by the creators of the Timesplitters franchise, not Crytek, perhaps an indication of why it feels lackluster compared to the excellent single player. On the other extreme Killzone 3's multiplayer is phenomenal. If you've ever wanted to know what would happen if Infinity Ward made Team Fortress 3, Killzone 3 is that game, minus the inevitable nuke. It's very class based, and coordination is important, however 80% of players haven't figured that out yet so it's still fairly simple to lone wolf it and do fairly well. There's a lot of depth, excellent maps and even some vehicles thrown in for good measure.
So, which games better? Neither. Championing one would feel a disservice to the other. It comes down to preference and console ownership, but these are the best FPS' I've played since Bad Company 2, and there two of the best looking games I've ever bought. If you own a PS3, I would suggest both, but until PSN is back online, perhaps try Crysis 2 first.