Both will be big no doubt! But Halo 3 will not have the same impact as it's predecessors... because it has too much competition this time and it doesn't offer something substantially different, why change a winning formula right? Who knows what other games will rival it in it's lifespan. Haze, Unreal Tournament, Killzone 2, hey even Metroid 3 at a certain extend!
The biggest competition has to come from Halo 4! Expect Bungie to listen carefully to the critique the game has already received when releasing the Beta, that the improvements on Halo 4 will make Halo 3 irrelevant! That will not happen to Mario Galaxy! Each Mario game is unique but for some strange reason the Mario games launched in the launch periode have always sold more than the sequels...
With an attach rate of 25% in it's entire lifespan I think 15 million is a reasonable number (Wii selling 60 million wich is a very low estimate). In the best case scenario with Wii selling over 100 million units, that number can easily double!
Let's say Xbox360 will sell between 30 and 40 million, Halo 3 will roughly sell between 10 and 12 million with an attach rate of 33%. Wich is in line with it's predecessors.
