Assuming NX is a console/handheld hybrid and releasing in 2016:
the new Zelda should be a game, playable on Wii U and NX. Game Disk for Wii U and Game Card for the NX are included in the box. Having the NX allows for better graphics and portability (with Wii U graphics).
This is also the reason for the delay I guess.
One can dream.
NX and Wii U versions will be sold seperately but for not much less prices.
Save data is obviously shared across disc and card.