Sorry guys, not trying to start any flame/hate in here, but me and my brother have been arguing about this lately, so vote for the game you enjoyed more. Don't be fanboy. Attempt to assess the games strength's and weaknesses, rather then just 'HERP HERP HALO' or 'HERP HERP KILLZONE'. If possible, I will lock the actual posting part of the thread, and leave a poll open.
Personally, I enjoyed both games thoroughly, but Killzone 3's story won me over in the end. I don't think I've enjoyed a games story as much as Killzones to date. Of course, that's not to say Halo Reach didn't feature an almost equally amazing story.
I'll edit my opinion further later, but right now I am rather tired. Cheers, and goodnight!