I voted HL2. The story's premise seems a lot more interesting, although both games look pretty epic... and the fighting itself, well, it's definitely going to be really different in HL2 whereas I think Halo will just be some slight additions here and there (to an already pretty good, but somewhat repetitive combat system.)
Halo co-op and single was what gave it power originally, so I'm kinda surprised they threw that off and focused on the multiplayer (which I heard is really good.) HL2, I'm not even sure if it has it's own multiplayer, but the sheer number of mods that will spring off of the awesome Source engine insures that you'll never get bored. You can't write mods off as another subject. They are part of waht HL2 represents, which is incredible possibilities with an incredible engine. Even if I got tired of CS:Source, there will be NS:S, DoD:S, and on and on, and this is something Halo 2 is not capable of.