Well, it's all subjective really.
I happen to think that MP looks better than Halo, due mainly to the fact that it's so much cleaner looking. Halo had some pretty extreme mip-mapping going on.
What game looks best is really just a matter of taste, but I do think we need to see some actual video and some higher quality pics to place Killzone ahead of either one at this point.
I don't doubt that it will be a nice looking game, but the lack of progressive-scan isn't an encouraging sign.