Xenoblade Chronicles X is not on the Switch.

Although it's likely it will be ported eventually.

3DS has a much better library of games for me. It helps it was supported longer and being the market leader for handheld means better third party support than WiiU too. 3DS by a landslide for me.

WiiU did have some gems like Mario Kart 8, 3D World... but they're all games I wish I didn't have to turn my WiiU on to play, and now with the Switch I don't have to.

I liked the WiiU, but I loved the 3Ds.

Xenoblade X is not on Switch and unlikely it will be.

Wii U has some of the best games ever made on their respective genre. 3DS has very good games, but none of them is particularly exceptional.

Wii U wins.

I still dont think the switch has quite lived up to the 3DS. Such a fantastic system with so many unique gems

3DS by far, tho I loved Wii U

3DS. The first-party line-up is better than the Wii U, and that usually makes or breaks a Nintendo platform.
3DS also has a much more unique library. There are hundreds of titles that have never been released on any other platform.
There's still plenty of reasons to get a 3DS in 2021. But the Wii U? Not as many.

When I think about it, when you line up many of the games or series the systems shared the Wii U does win out.

Smash Wii U > Smash 3DS
Super Mario 3D World > Super Mario 3D Land
New Super Mario Bros. U > New Super Mario Bros. 2
Super Mario Maker on Wii U > Mario Maker on 3DS
Mario Kart 8 > Mario Kart 7
Hyrule Warriors > Hyrule Warriors Legends - though I prefer the New 3DS version of legends since it gets rid of the performance problems,
Xenoblade Chronicles X + the original Xenoblade on virtual console vs an inferior port of the original came on New 3DS.
New HD Donkey Kong Country that improves on the last game in ever way compared to a downgraded port of the Wii game with worse graphics and half the framerate.
on.
Both systems had 2 Zelda remakes and 1 brand new Zelda and the Wii U wins out in all 3, with Wind Waker and Twilight Princess HD and Breath of the Wild

It's when the full line-ups are reviewed that the 3ds really starts to pic up in comparison, Wii U has no answer Fire Emblem Awakening, a game that might have revitalized the franchise. A shared port of Monster Hunter 3 is better in HD, but then 4 and generation were only on 3DS until the Switch version as announced. Professor Layton, Phoenix Writem Zero Last Escape, mainline Pokemon, Bravely Default, Metroid Samus returns. Kid Icarus Uprising These are games for which the Wii U had no amswer.

WiiU has Breath of the Wild, so it wins it for me.