In terms of animation, death sequences, lighting, geometry / polygons, etc, from a technical perspective Killzone 2 seems to stand far above what Halo 3 offered.
Possibly Resistance 2 will steal this game's thunder, or maybe Resistance 2 can help Killzone 2 and vice versa attracting many new FPS fans to the console interested in playing both games. I assume both titles will show their strongpoints, both seem have their own unique take on how to create the ultimate FPS experience.