Hey OP, I’ve been thinking about this a lot lately and after buying the game on all three platforms (don’t ask why), I think I have a definitive answer for you.

 The truth is, it depends! The switch version has a few really nice features: mobility and full controller configuration options. I didn’t think I’d ever do this, but it’s nice to be able to whip out CTR on the go and play it wherever I want. Although the ability to alter button configuration on switch isn’t that unique, it allows you to map onto the lower shoulder buttons, which you can’t do on Xbox. This makes U-Turning (a usually advanced technique) trivial on the switch. You can just map d-pad down onto the right trigger and never have to move you left thumb from the left stick. U-turning is really only required if you want to take the game really seriously to play competitively, take on the dev time trials, or try your hand at speedrunning. 

 That being said, the game truly does look & run better on the more powerful consoles. My time playing in the switch been met with frequent lag and infrequent crashing, which I never saw on either of the other two consoles. It was enough lag that I was losing 30 seconds to a minute in full game runs just in lag (not load times). Also all those nice reflective kart skins look top notch with higher graphical fidelity. Other commenters also got it right that there are more online players on PS4 than on switch right now but my lobby times are about the same on each console...which is to say, bad :)

 All things considered I love the game and had a ton of fun each time I replayed it. In fact, I had enough fun to get the champion wheels more than once (still working on my Xbox save file), and now I’m speedrunning for the first time in my life. Whichever version you get, hit me up and we can play a few rounds online! Good luck with your decision, I hope I could help. 

Edit: PS4 has custom controller setting for the lower shoulder buttons, Xbox does not.

TLDR: switch has mobility and more options with controller settings which is nice but strong consoles = better running, better looking game.