Wii U dev here, the dev kit allows you to use two screens. I haven't bought the kit yet, just browsed the forums. No I'm not violating the contract by telling you that, I'm just not allowed to disclose sensitive information (tools, manuals, etc.).
My advice: don't develop for the three big consoles unless you have a game that needs one of their controllers to function: for example, if you have a game that requires a stylus or touch screen, the Wii U is a good choice. The reason is that PC is the best platform, but it's dying like Sony, so you gotta choose which sinking ship you want to publish your game on.
Mobile is shit, nothing but greed-central, so don't give into temptation: making gamers pay for in-game items, putting content behind pay-walls, outrageous DLC prices, Patreon abuse, etc. is killing the mobile market faster than mobile device's own limitations. Sony is dying, Microsoft doesn't know shit about games, Nintendo is ruining its franchises, and Valve and EA are full of DRM. DRM is terrible for gamers: trojans and viruses, forcing people to rebuy games after they get banned, etc.
So go PC unless you have a specific controller in mind. And start making those CoD and Flappy Bird clones for virtual reality, because heaven forbid we don't have more copycat garbage when the virtual reality gimmick goes full swing this year.