Niether of these were that memorable for me. I remember them being solid but other than that nothing. So i refrain from my vote

this one is a much easier choice for me, both games are great, but Killzone 3 is clearly the winner.

Killzone 2, launch will be forever memorable. it set so many tech standards for consoles. and still to this day only a few games have surpassed it graphically (Uncharted 2, God of War III, Killzone 3, Crysis 2, Uncharted 3, God of War: Ascension, Crysis 3, and The Last of Us). thats only 8 games in nearly 5 years since it came out, and 6 of those games are sony first party). Killzone 2 had the better story telling than Killzone 3 (though i felt 3 had the better story actually, just not told as well). obviously KZ2 didnt have as good as Graphics as KZ3, but i prefered its moodier, darker, and drabber atmosphere to killzone 3. Also Radec was awesome. I didnt really get into its multi though.

killzone 3, is amazing. by far the best FPS graphics on console. improved so much on Killzone 2. the atmosphere, particle effects, explosions, facial and character animations and models were superb. gun models perfect. Snow amazing. its story was as well told as Killzone 2, as there were some corny moments. But the campaign was better, due to its increased length, much better gameplay, and awesome and many setpieces (snow mobiles, mechs, jet packs, spaceships, zero-G!!!!)

Did i mention improved gameplay, killzone 3 not only tightened the controls, and added set pieces and jetpacks to the gameplay. but it introduced Move controls. They were nearly perfect, and transitioned perfectly to the great multiplayer (which by the way killzone 3 has the best FPS multiplayer). With the move controls you can effortlessly pick enemies off with precision head shots. The multi also had an awesome class system. that were balanced very well. Also killzone multi had jetspack and mechs!!! freakin awesome. flying around in the jet pack mowing people down. then flying to a snipers perch, head shot a guy in a mech. take the mech, and wreck havoc!!

winner: Killzone 3.

My pick is Killzone 2. It had jaw-dropping visuals at the time, some very unique technology, great weapons, a great villain and just heaps of character. Even the controls, which were slated by many, added a very unique feel to the game. 

Killzone 3 opted to add a bit more colour and while visually it was more impressive, the changes took away from the grittyness of the first PS3 installment. In some areas I also found that the game seemed to have a few odd design choices and overall it was less polished than KZ2. The campaign lacked a cohesive structure, the story was all over the place, there were a few jarring technical issues and just so many missed opportunites. Certainly a major step down from the single player experience of Killzone 2. I was scratching my head a few hours in, wondering how GG got everything so wrong. There was some positive progression in this game over the first including improved diversity of environments and jetpacks...but I'm struggling to think of much else.