Halo 3 in my opinion is better than Killzone 2 in everything but the graphics.
Considering it was the follow up from previous two game it followed the story well. The story mode was good and it had co-op as well which made it a lot more fun. The online multiplayer you had from semi-customizing your characters to a large variety of maps, game modes, weapons, map maker. Vehicles You could play with your friends (local) online.
The multiplayer is very balanced. They even went far as to implementing competitive maps into the game for those who want to their skills to the next level. Bungie.net with the stats tracking and being able to look at the overview of each game.
I felt so much was missing from Killzone 2 despite being a solid shooter and huge improvement from the first game. There was just nothing that kept me playing Killzone 2 after the story and a few multiplayer matches. The multiplayer felt very uninspiring for me.