See my Nikon wish list.
In general I am not a fan of dual slots of differing types: in the field I’ve had problems with the camera switching to the wrong card when I have pulled the SDXC card for download, leaving me unable to download images without attaching a USB cable to the camera (because I lack the necessary card reader). And I haven’t always had that USB cable along.
In some ways, I’d rather have dual (slower) SDXC slots—the speed of XQD sounds nice, but the hassle of the card reader in the field isn’t appealing and XQD makes no difference to me in the field—well, that’s my assumption and perhaps I'm wrong so I should find out. Also, speedy download is a plus at the end of a long hike at 9 PM.
So given the reality of the Nikon D850 with its SDXC and XQD slots and in the name of testing camera behavior, I ordered the Lexar 128GB XQD with USB 3 card reader. Maybe I’ll change my mind if I just have that reader always at hand? I really need more than one, and it’s small enough to misplace easily.
The Lexar 128GB XQD by itself is discounted as I write this, but for $25 more I think its worth buying the Lexar 128GB XQD with USB 3 card reader so as to have the reader, especially if one doesn’t already have an XQD reader!
Micron is discontinuing the Lexar brand, but my Lexar cards have been highly reliable, and I don't consider this an issue—buy 'em and use 'em.