Things to remember:
Wii U is out of production
Wii U has all the 1st-party titles it will get (RIP)
Wii U will cease online gaming eventually, sooner than Switch
Wii U can emulate a lot more than Switch and you can play those backups while Switch gets releases
You can buy accessories cheaper
You can buy a refurb Wii U from Nintendo for $200.00
Switch won't have what you want for some time, depending on what you want
Wii U is out of production
Wii U has all the 1st-party titles it will get (RIP)
Wii U will cease online gaming eventually, sooner than Switch
Wii U can emulate a lot more than Switch and you can play those backups while Switch gets releases
You can buy accessories cheaper
You can buy a refurb Wii U from Nintendo for $200.00
Switch won't have what you want for some time, depending on what you want