I'm pretty sure that sound glitch is dependent on hardware, I didn't get it in the alpha and I didn't get it in the finished game.
Halo II defiantley has better multiplayer, (it actually HAS multiplayer), co-op story and more vehicles, apart from that I think HL2 beats it narrowly. When the expansion packs and mods come along, it'll be alot better. Also I found Half-Life 2's story to be badly explained, and they both had poor endings. I find Halo II to be more fun but Half-Life 2 to be the better game.