New Super Mario Bros. U is being released as a digital download for the game on June 20 in both North America and Europe. However, it's also coming to retail as a packaged standalone. It'll be available August 25 in North America, and July 26 in Europe. The DLC will be priced at $19.99, while the standalone is $29.99. You won't need NSMB U to play the packaged game.
This morning's Nintendo Direct also revealed that Nabbit, a little critter who you chased in the first game, is a playable character in the new content. He'll be available in multiplayer games as an easier alternative to Luigi and the Toads. He won't take damage when he touches enemies, but to balance that he won't be able to use power-ups.
Super Luigi U makes changes to NSMB U's campaign to make every course "all-new" - it's essentially a new Luigi campaign.