What do you think, and what's your reasoning?

3DS because Japan

Wii U... I kinda doubt a 6 month old port will beat it... even though its on 3DS.

3DS. cause bigger install base.

Wii U because many people interested in the game already have it and I don't see many people double-dipping, on an inferior port no less. In Japan the 3DS version will sell more but everywhere else Wii U will prevail.

Yeah, this is pretty much my thinking. Being late and gimped will hurt it globally, but the Warriors series is biggest in Japan, which is Handheld Central.

WiiU, the game is already a million seller globally so the 3ds version has a lot of catch up to do

The one with more potential.
3DS. 50+million units on the market.
Wii U. Less than 10 million units on the market.
Now if you were to ask about ratio, I think it would go to Wii U.

Wii U, for a number of reasons. a) It's already been out for at least 9 months (10 in Japan). b) By the time the 3DS version comes out, the Wii U version will have at least sold 1 million units worldwide, not counting digital. c) People will want to play the definitive version of the game which is already out.
If this were a situation like Smash Bros. where they released the 3DS version first, and everyone just wants to get their hands on it as soon as possible, than it would've been possible, but since that's not the case, I don't see the 3DS version, despite being on a significantly higher install base, outselling the Wii U version.

Probably 3DS. It has a bigger install base and I'm thinking maybe the reception by both the critics and market would more positive after they warmed up to the idea of a Zelda spin off.