well, halo is the better game, but halo 2 is still a good game.
loads more weapons, new gameplay features, upgraded graphics, online play.
the campaign was a ride, though the ending was disappointing.
while i still love The Maw and pretty much every halo level except The Flood and The Library, halo 2 has a lot of really good levels.
myself, i enjoy pretty much every arbiter level except The Great Journey, wasn't a great way to end the game. the Master Chief levels get really good towards the end i find, High Charity especially.
the multiplayer is great. way deeper than halo 1, though they could have at least put in the halo 1 maps as well. the new maps are nice though, i'll take Zanzibar over Rat Race any day. though i would like to see Chill Out, Sidewinder, and Hang em' High as download content sometime.
my only big letdown with halo 2 was the empty promises. melee combos, ATV hogs, the list goes on.
anyways that's my rant