During the Wii era the 3D and 2D Mario games targeted very different markets and both Galaxy and NSMB had incredible legs. I certainly bought both and felt they offered completely unique experiences. However, now 3D Mario is moving away from the single player platform/adventure genre towards a 'party platformer' which has traditionally been the niche of NSMB.
From what we know so far, I personally can't see the need for both games. Anyone else feel this way?
(Lets not even get in to why Nintendo thinks that in 2013 2D platformers and mini-game collections are the only genres needed to sell consoles.)