Prior to purchasing my D500, I rented one from BorrowLenses, but didn't bother with an XQD card. I just used the Sandisk Pro 300 MB/s UHS-II SD card from my E-M1 II, and never had an issue with it shooting in Continuous High drive mode. Granted, I was only shooting in ~3 second bursts, but never once did I fill the buffer.
Having said that, now that I own a D500, I shoot with an XQD and UHS-II SD card, with RAW files going to the XQD card, and JPEG files going to the SD card.
Honestly, unless you're filling the buffer on your camera, there's not much need to go out and buy an XQD card, unless you want to shoot with 2 cards for redundancy purposes.
If you write simultaneously to XQD and SD, your maximum speed is the slowest link, the SD card. It is unfortunate that for some reason Nikon elected to have two different slots in the D500 (and the D850), but there we are.
At any rate, to use a worn out automobile analogy, getting SD only in one of those machines is like getting bicycle wheels on a Ferrari because you only do the groceries. Why then do you get a Ferrari in the first place?
As for the price, there is practically no difference between a UHS-II card and an XQD. I only use one card in the camera, and it is XQD.