Well, I'm probably gonna get a lot of hate for this, but I prefer the Resistance games, so it doesn't matter to me if Halo goes multiplat or not. Now, let me explain why. If you don't care for my explanation, it's the blacked out part since it's off topic.
The story from the first game was one that I found very interesting. When I tried to play through the Halo games, I found that I wasn't as impressed. (I tried them after the first Resistance, but before the second.) Killzone 2 also had a good story, and I did enjoy the campaign a lot. Resistance: Fall of Man = Killzone 2 > Resistance 2 (approx) = Halo
The first Resistance has split screen co-op, which I still play every now and then with friends. I love it. The second Resistance has some intense 8 player co-op, which I play regularly. I played co-op on the first Halo, and it was very fun, but I prefer the Resistance: Fall of Man campaign so it wasn't quite as good, imo. Killzone 2? Well, there is no co-op. I'm a huge fan of co-op. Resistance 2 > Resistance: Fall of Man > Halo > Killzone 2
I'm not a huge multiplayer person, Warhawk being my main exception. I did enjoy the multiplayer for R:FoM, but never played it a ton. I've played some of the R2 multiplayer, and it's good as well, but still not my thing. Halo, I do actually enjoy the multiplayer, but I only ever play local games. I don't play much either, which is probably why I still prefer Resistance. Killzone 2.... Well, I just suck at it. I'm terrible. Awful. Dreadful. It's just pathetic, really. It's fun, but after getting annihilated with a KDR of less than .1, I kinda get frustrated. That's more my fault than Killzone 2's fault though. Halo = Resistance 2 > Resistance: Fall of Man = Killzone 2
Now that I've gotten that out of my system, I'd like to say that one of the reasons I'm not as big on the Halo games is because I've never had an Xbox or a 360. Maybe if Halo 4 was multiplat, I'd get into the series but I doubt it will be.
Disclaimer: I'm not saying that Killzone 2 or Halo are bad in any way; I think they're both very good games. Better than COD4. Yes, I went there. More hate, I know, but I never got into COD4 either. I just have weird tastes in FPS. =)