Assuming the Wii U version comes out first, I'll buy it on that. Then the NX version sometime afterwards.
I'll assume that the Wii U version will be rare due to Wii U not selling well, and the NX (hopefully) being the opposite.

Wii U obviously, because I already own that system

Wii U since I definitely won't be buying NX at launch.

Nintendo seriously needs to give us some new info on this game so we can actually have something to discuss rather than this kind of stuff. People are so desperate for this game that we're talking about which system we'd rather buy it for assuming it releases for a system that we know nothing about. Good lord.

Wii U since I have a Wii U. Wouldn't pick up an NX console in its 1st year though, I normally wait around 2nd or 3rd year. The only exception was the PS4 due to hype only.

3DS!

If there are two versions I might get both.

The GameCube version of Twilight Princess turned out to be a good collector's item that's worth more than the MSRP.

The NX version will almost definitely be an upgraded version, so I'll get that one.

Since I own a Wii U, i will buy for Wii U.
Also because it was the console it was developed for, with probably the gamepad in mind.