Well these games aren't even in the same category. One's a FPS, while the other is a musical type of game. If I had to choose, then it'll be GH3 because it's just a little funnier.
I would suggest Guitar Hero 3 because it has enormous replay value. But if you're looking for a good online shoot em up game to play with your buddies then go Halo.
Yeah, I agree because each game is for each game type of person. Also, before I chose Guitar Hero 3 because it's just funner, but now I think that I would choose Halo 3 because it has a better story to it and it's like an adventure and I just love adventures in games.