I have never actually played either franchise, but F-Zero for a couple of reasons. Diddy Kong Racing is similar to Mario Kart, and would come across to people as both more kiddish and more "spin-off" or repetitive (regardless of whether that's true or not). F-zero would provide more variety in the lineup and has the potential to look really good on the Wii U. Plus, I think it's good and perhaps necessary to keep making games of your major characters if you still want them to be known - Diddy Kong has already appeared prominently in Tropical Freeze, while at this stage Captain Falcon is probably known better for Super Smash Bros than anything else.

Personally I love WipEout, but F-Zero not so much. But the harsh reality is that the futuristic racing genre is dead and has been for a long time.
WipEout Pure - PSP - 2005 - 1 million
WipEout Pulse - PS2 - 2009 - 0.57 million
WipEout Fusion - PS2 - 2002 - 0.25 million
WipEout 2048 - PSV - 2012 - 0.32 million
WipEout HD Fury - PS3 - 2009 - 0.06 million
http://www.vgchartz.com/gamedb/?name=wipeout&publisher=&platform=&genre=Racing&minSales=0&results=200

F-Zero X - N64 - 1998 - 1.1 million
F-Zero: Maximum Velocity - GBA - 2001 - 1.04 million
F-Zero GX - GC - 2003 - 0.65 million
F-Zero: GP Legend - GBA - 2003 - 0.16 million
http://www.vgchartz.com/gamedb/?name=f-z&publisher=&platform=&genre=Racing&minSales=0&results=200

If Nintendo make F-Zero it will be pure fan service, as Starfox probably also is, and it will sell worse than Bayoneta 2 or Wonderful 101. Diddy Kong Racing would give them money back at least.

F-Zero, easily.

F Zero no contest. The Wii U already has a party racer, now it needs a high octane racer.

Diddy who?

No, clearly F-Zero. DK Racing was nice back in the N64 days. But that was mainly because of the single player. Multiplayer never was as good as Mario Karts.
And Wii U already got two good fun racers with Mario Kart (espacially MP) and Sonic Transformed.

F-Zero is something we haven't got in a very long time and something, no other console has of now. And today it could have great single player as well as online mp.

... seriously? The same Wonderful 101 that's sold 270k in 16 months?

Wipeout HD sold quite well digitally though, but the F-zero franchise is dead as a horse. Nintendo refused to make one for the wii and has stated there isn't a F-zero down the pipeline (neither do they have plans for it). The wipeout developer doesnt exist anymore (they did make a new wipeout on ps3 before they where shut down) so this genre is indeed as good as dead. Perhaps another Sony studio will make Wipeout (evolution studios) but that's going to take a while if it happens at all.

I would buy it and so would you, but I can not see demand for this kind of game anywhere! This genre is VERY hard-core.

Evolution Studio would be great. Sadly MotorStorm, that would be good as inspiration for a new WipEout, has lost in popularity as well. Maybe most people do not like arcade racers anymore.

I'm not saying it would sell gangbusters, but I have a hard time seeing it selling less than Wonderful 101. At the very least the hardcore Nintendo crowd should carry beyond that.