I've played both Killzone2 and Killzone3. The latter's story is somewhat lacking, and even dull compared to its prequel (story-wise), however, what it lacks in storytelling delivers in multi-player. The role-system is well defined, and it's more easier to ascend in one role than it was in this game's predecessor.
In Killzone2 you started off as a ordinary private (no special abilities or weapons), and by gaining more exp. points, you could unlock special roles starting from the medic over the engineer etc. to the marksman. In Killzone3 you can choose your specific role from the get-go, and by gaining experience points you can unlock weapons and advanced weapons and abilities for your favorite role.
The game modes are the same as in the second installment with one additional game mode. Guerrilla mode is your team-death-match, Warzone is a mixed, plant-bomb/defend, assassinate/protect, team-death-match, inverse-capture-the-flag (with one flag) game where the end of the timer, or achievement of a goal triggers the next round. In operations one team is on offense and the other is on defense (unlike in Warzone wherein these roles swap eventually). The maps are diverse, and fairly balanced, which some of them contain special vehicles like walkers or jet-packs.
Final words: if you've played KZ2 and concerned exclusively about the story part you may reconsider buying this, but if you're in for the multi-player it's a must have!
For more info on roles and etc. you may visit the Killzone homepage...