I personally prefer Killzone 2 more than the third. First of all, the menu. The menu in Killzone 2 fit to the "destroyed and war-ish" theme. It was easy to access, and looked over all very good, and good colours of choice. Killzone 3's menu is however, amazingly ugly in comparison to the Killzone 2. It is red... Simply red, the music played while browsing is awful, and it is just so away from "killzone" it could possibly get.
Second of all, singleplayer. I really enjoyed and loved with all my heart the levels in Killzone 2 same as the movement. It was hardcore, tough and it made you really get that war is a bitch. The variations wasn't many, and for the record, I liked it. The campaign was dark and filthy. I believe that Guerilla Games listened a little too much at the fans when they were pledgin and begging for variations. Now we got it, a jungle in Helghan.. hm, really? Sorry, but that is just a big no for me. Helghan in Killzone 3 doesn't feel like Helghan. The movement was also something a lot of people disliked and compared to the Call of Duty series. I think the movement was the most realistic in a game this far. Think about it, you have a buckload of ammo, guns, grenades, etc on you. Of course you move that slowly! In Call of Duty: Black Ops (for an example) you can run faster than Usain Bolt. It feels a little to fast for me. I am young, and I haven't been in a war (obviously) but running as fast as they do in 'blops' with all that ammo (,etc) on you doesn't feel realistic. Any how, this isn't about Black Ops, but Guerilla Games truly have cod-ified the series, which according to me is a shame. Graphics isn't such a big part of the game, but you have to say Killzone 2's graphics rocked. Killzone 3's graphics however is notably not near as good as they were in Killzone 2, that because of the 3D technology. I mean, a majority (something as 97 %) will not play with a 3D tv, I think it was pretty unessecary to add it in, also as it made the game look worse. Now multiplayer, I admit.. multiplayer wasn't the strongest part in Killzone 2 but it was amazing. It wasn't like all the other games in the market, it stood out. Now, I haven't actually played multiplayer in the full version of Killzone 3 but from what I saw at the beta, it is cod-ified. Points popping up in your face when you kill someone? Perks?
Needless to say, I think Killzone 2 is the better game. Now I can only hope Guerilla Games get back to the good times, in Killzone 4.