A:Answer Release date is Januaary 11!
Deluxe version:
In addition to simply looking better, there are a number of changes and improvements from the original New Super Mario and Luigi U games.
One of the biggest differences is in the character roster. Total, there are five total playable characters: Mario, Luigi, Nabbit, Toad, and Toadette. The Toad you can play as is "Yellow Toad" from the original games, and Blue Toad appears to have been removed and replaced with Toadette.
Toadette is an interesting addition, as she can transform into a character called Peachette by picking up a Super Crown in the game's levels. This gives her Peach-like abilities, such as floating. She's a great character for new players to pick up due to how hard it is for her to fall off the stage. The game also includes a Hints gallery if you find it difficult to make progress, a Super Guide that will finish levels for you, and reference videos for beating levels.
For those who find the game too easy, however, New Super Mario Bros U Deluxe includes three new modes: Boost Rush, Challenges, and Coin Battle that offer more complex challenges.
Finally, Luigi fans can rejoice, as all the DLC content from New Super Luigi U is also available for free, on launch.