Killzone Multiplayer isnt as good as Halo multiplayer, mainly because of Forge and the community bieng larger and more varied.
However, everything else in Killzone games trumps Halo easily. I own a 360 and have Halo 3 and Halo reach, and played only a couple of hours of both of them, because they are lame. Controls are archaic, graphics are plainJane and while the story is great, i just could never stick with it because the games themselves feel dated and arent very exciting in singleplayer to me. Even coOp, which i love, has been uber dull and repetetive in Halo for me. They are not bad games, just not nearly the best shooters ive played. Resistance and Killzone are much better to me imo.
Killzone Multiplayer could step up to at least the scope of Halo (but probably not the popularity) if they include a map maker, more maps, and expand the modes. Personally i feel sony should skip the whole Dust514 thing and put out Killzone on PC, and implement cross-platform MP for KZ4. That would be a way to compete with Halo, popularity wise, but as far as the games themselves, i much prefer KZ series. Whatever the case, some improvements are needed, i would like them to expand to larger maps again. Playing KZ3 mp with the sharpshooter is quite awesome tho, a unique experience.