Brawl is honestly a better and more varied game tbh. I’m not a competitive Smasher, so my opinions may differ, but just looking at single player content, Brawl knocks Wii U out of the water in every way. Wii U’s modes are mainly just multi-man battles, like Classic has you chose between tons of multi-man battles, and All-Star is nothing but Multi-man battles. Classic mode had a better structure in the previous installments, like there was break the targets in between to lower the repetitiveness, there was also set places for big and metal battles unlike the random chance in Wii U where you get frustrated as the character knocks off a piece that has customs, which you want.
As for the extra modes, can that angry birds clone, Target smash, really hold a candle to the traditional target tests from Melee and Brawl. The Home-Run contest and Multi-Man are the same in both games so no comment. Also Trophy Rush isn’t nearly as fun as Brawl’s shooter mini-game, it just added even more variety to Brawl compared to any other Smash.
Brawl also had the Subspace Emissary, which added so much variety, as Brawl opened up its platform in roots. It was an 8 hour campaign with plenty of content, while Wii U’s big mode is Smash Tour, a board game that has the potential to be fun, but no one knows what they’re doing, and it felt lazily put together.
Custom Moves were awful in Smash Wii U though. They should have been easier to unlock as they are part of the characters, not something like trophies. It isn’t fun to unlock customs as they are completely random, so you have to rely more on luck than any kind of skill to unlock the things, doesn’t help that there are repeats.
I find Brawl way better than Wii U as the single player content was stronger and way more varied in Brawl. The game may not have the best competitive mechanics, but it was way more ambitious than Wii U, so I definitely preferred it more.