Ill have to say without online play, MP3 will have a rough path laid out if its trying to compete with Halo 3. Besides
having multiplayer, it will also be the last one to grace the franchise. With that said, people will be swarming to it just to "Finish the fight" and see how the series will end.
Knowing it wont be the end for Metriod, it wont have as much of a 'gotta have now' feeling.
So in terms of "Who will sell the most" I will safely say Halo 3.