In terms of software quality, which transition did Nintendo handle better, 3D or HD?

[Leynos]

Wii U had top quality games 1st party wise but HD gaming as the standard was 7 years old. It wasn't new anymore and a lot of the kinks were worked out of the early days.  So they were not revolutionary/transformative when Wii U came out.  3D games as the standard was still in peak growing pains when N64 debuted. Mario 64 set such a high bar and became the model for how most 3D games were made. Transitioning 2D games to 3D was a difficult hurdle at the time. Nintendo did it nearly flawlessly when few did by 1996 on a home console.