I agree that Halo 3 will be a very good game. I've enjoyed the series alot, but like all first person shooters it will soon be overcome, or the developers won't be able to keep up with the fans.
Some of the FPS that I've enjoyed have been the GoldenEye for N64, HL2, Unreal Tournament GOTY (always a fun game at LAN parties), and Halo. I've played Gear Wars, but didn't think much of it. I still don' t have a 360 or PS3, but if MS can get their 360 into decent shape, (since they just recalled most of them for .... what was it? $1 billion in repairs?...), by the time Halo 3 comes out, I'll probably get that over the PS3. (two of my friends have the PS3 already, and another the Wii, they are fun systems.)
Edit -- being busy with college though, I've been out of the game loop, so I've never heard of KillZone...
Edit2 -- just watched some trailers, if the cinema screens are that good, and the game play looks as good as the short chips they did show, it should be a nice looking game anyway. Can't tell about controls or story line though.