The head-to-head match-up, put together by GameXplain, compares video capture from three games: Super Mario Bros. 2, Super Mario Bros. 3, and Kirby Adventure. Based on the evidence presented, the Mini-NES seems to render the games at higher quality as well as more accurately, with options to output the game in 4:3 ratio or Pixel Perfect, a mode that shows the games as they would have appeared on PCs while in development.
As you can see, the Mini-NES output (left) is much sharper and more vibrant than the Wii U virtual console version (right). The Mini-NES also appears to have much better sound quality as well.
This comes on-top of news yesterday, both good and bad, about some of the Mini-NES's other features, including the terribly short cords that the controllers come with. You can watch the full video comparison below.