Let me supplement the "2D Mario vs 3D Mario as system sellers" debate with this:
2D Mario games sell on the strength of their legs. 3D Mario games are much more front-loaded. So 2D Mario doesn't push a lot of systems all at once, but it becomes one of those games -- you know, when people say "when it has X games I'll buy it." 3D Mario, on the other hand, makes people say "it only has 1/2 X games, but FFFFFFFFFFFFFF I NEED IT."