I only remember one opening score for Landfall, and that was the same music used in the High Charity level on Halo 3, so I assume it's Marty's.
My issue with with Williams is that he can definitely write a good theme, but everything in between the themes is totally forgettable. If you listen to his soundtracks, you end up sleepwalking through an hour of music and all you remember is the couple-minute theme. Obvously, there are a few exceptions, but that's my general experience listening to his scores.
Well, I've always been a massive Star Wars fan, and I can listen to every single piece from the soundtrack and not get bored. But that's just me, I'm a tad wierd like that.
And I didn't think about the guy who did the music for the games... seems a bit silly in hindsight, but I agree. If the movie doesn't reflect the atmosphere of the game, it probably won't work well.