I’ve played Halo on X-Box, but not online. I have played UT2004 online, and I’d say that UT, on the PC, multiplayer, is the superior game. More game modes, better graphics, more weapons, and I believe more players per map. Also, UT has one of the biggest and most inventive mod communities out there. If you just want mindless, unrelenting action, UT is the game for you.
Personally, I’d rather have a plot, so I prefer Halo. Plus, the X-Box version let you play through the campaign in co-op multiplayer mode, which was more fun than I’ve had from any FPS since… well, since Doom, I suppose. Can’t effing believe they stripped that out for the PC version. What were they thinking? I’d have bought a second copy of the game just for that. Idjits.