I have a wii u and xbox one s. For homebrew wii u was a doddle to install and runs retroarch fairly well as well as backups etc. Plus n64 injects are awesome like castlevania 64 etc.
Xbox one is pretty much retroarch only for me and it can run ps1 games fairly well which wii u cannot. It has other things like ppsspp but not sure if compatibility is that great.
However, if I had a switch I wouldn't bother with wii u unless you need gc and wii games.
I think I read the switch has a decent standalone Amiga Emulator whereas retroarch for wii u and xbox has p-uae which is complete and utter crap and the most pain in the ass emu to set up. Plus cannot map keyboard to pad. Grrr.
If you are prepared to buy games get xbox, for wii gc backups get wii u.