I don't mean to promote my own article, but:

"While trying out Wii games on my new Wii U, I noticed my set was reading the signal as 1080p. The game I tested appeared to look a bit sharper and more vibrant than I recalled. Making sure I wasn’t just falling for the excitement of a new console, I decided to compare some screens of games running on both the Wii and Wii U.

Wii:

Wii U:

Note my set reading the signal as 1080p.

Wii:

Wii U:

Note more vibrant colours and a vertical stretch to fill the black bar at the bottom.

Wii:

Wii U:

There you have it. I’m no tech wizard, but I do have a theory that HDMI may just inherently have higher quality output versus Component, but who knows? So whether or not the Wii U upscales Wii games is still up in the air, but I do know these screens objectively look better on Wii U."

http://www.consolexp.com/does-the-wii-u-upscale-after-all/

I played some Mario Kart Wii to test the Wii features of the Wii U.  MKW did look better on my TV through the Wii U than it does on the Wii.  I believe that this is in part to the fact that the Wii U upscales to 1080p as does me TV, but my TV sucks at upscaling anything below 720p.  I believe that the Wii U just does a better job at it than my TV.  It's not a huge difference, but anything is better, right?