Super Smash Bros. fro the Wii U is very similar to Super Smash Bros for 3ds. I will just cover the new features of the Wii U version.
I would say the first main difference are the stages. I won’t explain them, but they feature stages from home console games, like Mario Circuit from Mario Kart 8, or Skyloft from Skyward Sword. You get the point.
Smash tour is a new game mode for Smash. You play as your Mii, but not a Mii fighter. you go around a board and collect “fighters,” which are different playable characters. You will battle other players as one of the fighters you collected. There are board items and Battle items, but I won’t go into detail on that.
8 Player Smash
8 player smash is for people who think 4 people aren’t enough. with up to 8 players, this mode can sometimes be insanely hard and competitive. only some stages are available, but surprisingly Final Destination is not on the list. They added a stage called Big Battlefield, which is battlefield with more platforms and is wider.