IGN may have released a video, but supposedly their pictures of the Wii game were rather blurry and didn't represent the game right. So here's some clear comparisons showing the differences between the Wii and Wii U titles.
Grass Level
vs
Desert Level
vs
Snow Level
vs
Tower
vs
Water Level
vs
Castle Level
vs
Ghost House
vs
Finally, forest level comparison:
vs
As you can see, New Super Mario Bros U looks much, much better than New Super Mario Bros Wii in pretty much every regard. The backgrounds are nicer, the tilesets are ten times more detailed with better textures, and all and all, the Wii U game definitely looks like a next gen title compared to the older Wii game.
Does this make you think New Super Mario Bros U is even more impressive?
Grass Level

vs

Desert Level

vs

Snow Level

vs

Tower

vs

Water Level

vs

Castle Level

vs

Ghost House

vs

Finally, forest level comparison:

vs

As you can see, New Super Mario Bros U looks much, much better than New Super Mario Bros Wii in pretty much every regard. The backgrounds are nicer, the tilesets are ten times more detailed with better textures, and all and all, the Wii U game definitely looks like a next gen title compared to the older Wii game.
Does this make you think New Super Mario Bros U is even more impressive?