All I can tell you is that Killzone is a decent series. By far one of the better looking games in this generation, maybe only beaten by Uncharted or Crysis. Decent multiplayer, but forgettable once something else comes along.
Halo as a series is one of the most popular shooters nowadays. People may argue it's not innovative or that the story of the games isn't that great, but it has spawned an entire universe between all the different mediums. Whether an individual thinks that the story is good/bad doesn't really matter as long as there is depth to the universe.
The multiplayer in Halo is by far one of the best, and Reach is the orgasmic climax of what Halo can make. While multiplayer does drive the experience, the story in Halo CE is only considered better by some fans because of nostalgia and the fact that it introduced the world. The writing and character interaction was superior in Reach, even if it lacked the wow factor of the others. It still is no best work of science fiction ever, but above average.
Comparing and contrasting is hard to do since 90% of the people doing it are extremely biased one way or another. I'm biased in the way that I prefer Halo over many shooters [story wise I still like Half Life 20x more] but that does not mean I let my console choice get the better of me when I try other games. The amount of things one can do in Reach's multiplayer, not only serious matches but community games, making levels.... hell one person made a fucking quidditch game type, just makes it a more bang for your buck.
A Pimp Named Jeezy Creezy: Lasting all the way up to Valentine's.
And the Pimpeth lord sat amidst his real ballin' players, the Apostles, and raisingeth his cup of Cristal as they devoured the grapes and cheese from the finest bitches in town he said
"Sleepery Dee, Sleepery Doo.
Whoever betrays me tonight, I'll mo'fuckin' cap you!"