First and foremost, it depends on what types of games you want to play.
Platformers no longer interest me and I'm not a fan of the Mario characters and design aesthetic. I like racing sims more than kart racers and I much prefer 3D fighting games over 2D. The Wii U does have games I want to play but none of them are vital to me. By the time Halo rolls out, the Xbox One will already be a better second console for me, relative to my individual tastes.
Objectively speaking, I'd also say that the Xbox One/PS4 will probably eventually make a better secondary console. The main reason for that is this year's E3. It's convinced me that the total library of the Wii U is going to end up with less exclusive content than even the Xbox One, as they both will live much longer life cycles with longer effective support. Moreover, the PS4/XO will probably have more variety in that exclusive content, given that Microsoft seems much more focused on first party games than ever before.
Honestly, with games like Fire Emblem and Pokemon, the 3DS is a much better choice as a Nintendo second console than the Wii U.
The wild card relative to the Xbox One, however, is the PC. Owning a gaming PC does mitigate the need for an XO to some degree. I'll eventually buy Ori from Steam, as an example.
