Halo 2 had the most awesome online play in the XBOX. I remember asking my friends "do you like Halo 2's story?", and they were like " oh no, I had never played the story mode, I just play the online mode cuz it's so f***in awesome". And because of that there will be a lot more online players in Halo 3.
But Melee was one of the best kicking ass games in the GameCube, and think of that game in online, it would be crazy.
I think both games in online will be great.