Also the Wii U upscales the native Wii 480p to 1080p very nicely. On my Sony 75 HDTV, I test out various Wii games on vWii both at 480p and 1080p. The 1080p do look much better in overall quality. It could also mean my Sony 480p upscaling sucks as well.
Personally, i find Wii U games look gorgeous at 1080p, but for Wii mode, 720p seems to look best. Actual Wii doesn't look great at 480i or 480p, but emulators and VC look good at 240p (the main reason I see to keep an original Wii around after Nintendon't got good).