About 2D vs 3D, I think the 2DS sales stats will tell us what the customers really want.
But, I hope that 2DS will be a failure because I do love the 3d of my 3DS.
Anyway the 3DS was selling really good before MH4 release; and don't forget that the best seller on the machine is still one of the rare game (mario land 3D) where the 3D is really helpful. Maybe a coincidence, maybe not.