Well, userbase definitely has a hand in it, but how much influence it has, I've got no clue.
But I think darth is right when he says the userbase is more diverse. After all, the 360 is generally a more favorable console for people who play a lot of multiplayer games, and a lot of the multiplayer games out there are shooters. This could explain the huge numbers for the Halo/Gears games compared to the shooters on the PS3.
Buying habits are of course a part of it too, and since there is a proven difference in mean age on each of the consoles (http://www.gamedaily.com/articles/news/wii-attracting-young-boys-older-women-in-droves-ndash-nielsen-/?biz=1), where the PS3 has the oldest userbase (in the US at least). This is bound to have an effect on buying habits.







