It is still in 4:3, and I've seen people say it is still only rendered at 640x480... which is disappointing, it would be cool to see some basic upgrades like widescreen and a higher native resolution. Does anybody own both versions and notice a difference?
Having played both digital versions on Wii using component cables and Wii U using HDMI, there is no discernible difference. The Wii U version has additional support for the Pro Controller and the GamePad.
Virtual Console games are direct copies of the original game, emulated on the Wii U. The only improvement you may or may not notice is the fact that Wii U uses digital video signal (hdmi) as opposed to analog (RCA connectors).
The Wii ran 64 games at 480p through component cables, as the N64 could if you had a set (they'll set you back a pretty penny, as Nintendo never licensed 3rd party component cables for SNES/N64/GCN). HDMI should be a big improvement on an HDTV if you were running your Wii on composite(analogue) cables. Probably won't see too much improvement on your set if you were using component cables for your Wii, but there's been some discussion that the Wii U should emulate the titles a little smoother than the Wii did.
I have both versions, and the Wii U renders at 960x720 (The 4:3 version of 720P.) There's no edge antialiasing, but there's definitely additional texture filters that were not present in the Wii edition. It's tough for the average person to see the differences, but they are there. It's apparent if you compare the game inside the castle & examine the floor tiles at an oblique angle.