Rough google translate:
New characters in Legends are available for Wii U when buying Legends
Only the characters will go to the Wii U version, no maps, modes, weapons and other new features.
all DLC characters from Wii U are available in Legends, but you won't get the DLC characters for Wii U with Legends if you never bought the dlc, only new characters are added. So if you want the same roster in both games you'll have to buy the Wii U DLC that is out now.
Overall this means that if you don't have HW for Wii U, it's cheaper to buy the 3DS version because it will probably have more content than the Wii U version, not sure about adventure maps and boss challenges if that is includes though, I bet it is.
If you already have HW for Wii U you still can expect loads of new content. I'm especially pumped about that if all adventure maps are in, we now will have missions for all characters, which didn't happen in the Wii U version because how DLC is handled in JP for the game.