Kevin,
I sold my D810 to get the 850. I love it, the focus is awesome, and with the battery grip, I can get 9 fps. I do not shoot the reduced file size as I want the 45 megs for prints and cropping if needed for the books and magazine articles I do. I did not have a chance to see a D850 in person before I ordered mine as the nearest camera stores are in Albuquerque. I have been using Nikon since 1961 so I knew it would be good.
I think Nikon did the dual slots as only Sony and Lexar are making XQD cards and the future of Lexar was (and is) in doubt. I got a 128, two 64s, and one 32 before Lexar sales stopped. I stopped using CF cards in my D810 as they were not fast enough and too bulky.
Too icy to walk the high line right now. I am 77 years old, so my days of hiking up the hill sides for railroad photos are over...SMILE