900th post in this thread or whatever to say this but - High end gaming PC will give you access to 90% of xbox/ps4 games and depending how good your rig is , they'll look better too so as always it comes down to exclusives.
Complimenting that PC with a switch means ... well, Nintendo games. Breath of the wild , mario oddysee and the current best versions of Pokken tournament, Mario Kart 8 and more than likely a deluxe port of Smash bros wii U , Bayonetta 2 , wonderful 101 for those that skipped the wii U. Other brand new exclusive titles like Xenoblade 2 , probably some kind of zelda sequel down the road and any other typical Nintendo stuff you might be at all interested in. This stuff will never come out anywhere else.
Grabbing a PS4 then, means you'll have access to sony's first party library of content - Gran Tourismo , Horizon , Uncharted , Spider-man, God of War , Days gone, Detroit , Ratchet and clank , etc. These games will also be glued to the platform so it just comes down to which sort of titles you want to play.
I suppose pricing could be another factor - A switch costs less than a PS4 pro but due to the choice of flash rom based physical games the games could average a higher cost. The switch is also a portable while the PS4 pro is obviously not. Gaming on a PS4 will also cost you 70 bucks extra a year to get PS+ to have online play while on Nintendo I don't feel their upcoming service is really required at all (unless you really want to play splatoon or whatever) and even if you did , it's still far cheaper than sony's service.
If this were my decision , I'd buy a switch over a PS4 but then I'm also a believer in using a dedicated device to watch blurays with. One thing is for sure , you really don't need an xbox if you have a dedicated gaming PC unless you really want to play Halo.