Metroid because:

- Wii U has already a great "racer" with mario Kart
- F zero has always being a niche game and won't succeed commercially, the game that still can sell Wii U is a 3D metroid.

EDIT: Retrospectively Retro really should have done a Metroid in 3D instead of a 2D Donkey Kong. T'was a big mistake notably in US where a Nintendo "first person shooter" is really needed.

If i didn`t had to choose, i'd gladly say both, but seeing as i have to choose, i choose both anyway! :p

Metroid it is!
Just love Metroid more.

F-Zero would play very well on the Wii U, but Metroid would probably look amazing.

This!

F-Zero could be handed to Shin'en (If they like to be associated with it) and maybe be an eShop game, with DLC every other month. 

wipE'out, that is a more known game, is dead a long time ago, and so is F-Zero. It would need a massive hype and marketing effort to sell even half a million.

I wouldn't be able to choose, because I would really want to play both. Nintendo ought to just make both. They have nothing to lose.

That makes sense, if they would have released Mario Kart and Metroid in 2014 then Splatoon and F Zero in 2015, there would have been two years with a shooter (whether first or third) and two years with a racing game. Donkey Kong could have honestly waited and we would have had 3D World for a platformer in 2014, and moved Mario maker to 2016 for Donkey Kong to release 2015 (trust me, no one would have been VERY upset by this though I do think it will be a fun game).

Then, due to the games releasing in 2015 are a brand new title and a niche racing title, they could throw in Star Fox, Legend Of Zelda, and Xenoblade. So we'd get the tried and true heavy hitters in 2014 (with Mario Kart, Metroid, 3D World, and Smash), and a bunch of great smaller franchises (along with The Legend Of Zelda to keep the masses on the hype train) in 2015, it would have been a diverse schedule with some all around great releases.

As of now though the cards are all pretty muched stacked in 2015's and 2016's favor. 2015 is going to have all the smaller franchises mentiones above anyways (with Fast Racing Neo as a stand in for F Zero, which I think will honest draw about as many sales as F Zero probably would), but I'm pretty much expecting the next TRUE 3D Mario game to be announced at this E3, Metroid, Shin Megami Tensei X Fire Emblem, Devil's Third, and hopefully a few surprises from third parties. (Looking at you Crystal Bearers Wii U, or I don't know... Localize Dragon Quest X and Fatal Frame!!!!!)

I love F-Zero but this still isn't even close. Metroid of course!

Metroid, but I'd love both... as long as they're up to standard.

I choose F Zero because I can't play Captain Falcon on Metroid :P

Personally, I say Metroid since there's nothing like it on Wii U and it's my favorite franchise, but either F-Zero or Metroid would look so stunning on that system. It's been 11 years since the last console F-Zero, so maybe it deserves it more. Not sure either one would sell big numbers, tbh.

I think if they announce Metroid at E3 2015, throw in the big 3D Mario game, maybe add an Advance Wars or Golden Sun game (hopefully), and set SMTxFE all for 2016 release, that will give us a pretty incredible year for Wii U to sort of cap off the lifecycle. Probably wishful thinking, though.