Both. I don't think that the Mario series would be complete without one of them. I hope that they will make more 2D games like NSMBDS and -Wii for the 3DS and the next home console and they will not remember the genre again after a decade or something.
Regarding 3D platformers, there is no doubt that more 3D games will be released in the future.
generally, although tranditional 2D series
I think Nintendo want to carve out the market of other hot genre like 3D and RPG
also in everyside they got nice scores than other companies
I usually don't compare different types at one time
If I'm playing the games by myself, then I choose 2D, since I haven't completed any of the 3D games, pathetic isn't it? Although, I prefer watching LPs of 3D Mario games rather than the 2D games. So I vote for both.