It all depends. I have both, Wii U has a great array of games available to it when you mod it, which makes it my go to console for many games semi current and retro, without what seems as much risk for a ban from Nintendo. The requirement for the gamepad can be a bit tedious, but it's still a good console imo, and removes the need for having others. The Switch is nice, but handheld mode gets toasty and even a little uncomfortable while playing BOTW and other titles, which brings me to my next point. Many of the titles are available on Wii U, and currently there's not many Switch exclusive titles that make me feel it is worth the upgrade yet. This is Nintendo's flagship console, and while they'll continue to support the 3DS/2DS line, there probably won't be many first party games coming out if any, which means the Switch will be the console to receive games for the next 4-5 years. They may do revisions to the console itself, but I don't see them releasing a different console any time soon, so I'd expect to see some great first party games along with third party. And given how well some ports have worked like DOOM and the fact Nintendo is working with third party to allow the ports, I'd expect to see some other games ported from previous generation consoles/steam.
So in short, unless you're a die-hard Smash fan or fan of some other Switch exclusive, it's not worth it yet. It will be in the near future though since it will be the focus of their game releases.
Plus a lot of great options are out there for playing online banned, changing themes and layouts (still early in but works great, but a little finicky sometimes), and them clicky noises
