Killzone 2 appeals far more to me personally, both for single player and multi-player. Other games like Warhawk (being able to fly in aircrafts) and Resistance 2 (8 player co-op) bring additional interesting teamwork multi-player aspects to the table.
Honestly, I think if Halo 3 wasn't a Halo game, but launched with other characters, name and more a standard level of marketing the title would have been rated much lower. I expected more from this game compared to earlier installments.