I only have Halo 2, so I borrowed Halo 1 from my BIL. I've worked a good way through the campaign so far, up to the point where I'm on the Covenant cruiser trying to rescue Captain Keyes.

Differences in weapons between Halo 1 and 2: the rifle in H1 goes full auto, which I don't like. The 3-round burst in H2 is much better.
I like the scope on the pistol in H1.
I haven't used many covenant weapons in H1, but I like the Needler - unloading one magazine in H1 kills an Elite, whereas in H2 it took two magazines.
It's taken me forever to get used to not being able to double-wield.