RuRo on 02 January 2015
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.
