It depends on how the demographics for this gen of consoles pans out. If you look at game ownership as a bell curve by age, you're going to see a gradual slope upwards until about 20, and then a steep drop-off after 25 or so. Your center around that peak is more likely to buy Halo. Everyone else is more likely to buy Mario. Now, it's all but a given that the entire curve will be more even this generation, which bodes well for SMG, but because "peak" players are more likely to buy Halo than "tail" players are likely to buy SMG (for the casual gamer, there is no "must have" game), the curve has to even out a lot for relatively little headway.
A similar issue is the point that Mario is currently building up from a low, where (and this is my opinion) Halo has peaked already, and will in all likelyhood not be as defining a franchise as it was last generation. It's the "ascent vs. decline" issue. I actually think this one is more telling. Halo is a strong franchise and won't be falling too much from aforementioned peak, but Mario has the possibility to be as popular as the franchise ever was, which could imply the best-selling game of this generation.
I give it to SMG.








