While an average smash player will consider everything, stages included, when it comes down to deciding which version to pick, the average smash fan will consider prices first. A teenaged boy or young adult in the American economy will pick the 3DS version over the Wii U for it's mere price, considering that the 3DS is cheaper. At the moment, a good decision is to buy the 3DS version, as it has nothing that the Wii U version doesn't have. However, in the coming months, we will see more features arising for the Wii U, compelling the previously mentioned smash fan to buy both, or even consider buying a Wii U. Smash Wii U will probably have a Wii U bundle, even settling the matter.
Going back to stages, as we've never had to choose between two versions before, the mentality of a Smash fan would be driven towards modes and price, as the psyche of one fan is going to think of a stage as a 'playing field'.
I've gauged the public in my area on this, modes and price sell it for them. Obviously some will value other things, as you have stated. You will go for stages, while 60-65% will go for everything else.
So, the stages will sell, but only after coming after modes and price. I do admit in my previous post I haven't been clear, and I apologize for that.
I swear, that's my Achilles heel. Clarity.