Little late to the party....
Firstly I noticed the original post of D500 v's D850, so lets compare on a practical underwater lets go diving mind set.
Bear in mind I use the D800 and will not part with it unless a camera knocks it out of the water, so much so it would be worth upgrading. I housed a D90 and wanted to house a D700 and was told not to bother so you kinda get where I'm coming from.!
D500 is cropped D850 is full frame, so collected choice is to change all your lenses etc like i did. Read not above. Never looked back big expense but worth every penny.
I noticed the Battery life on the D500 is shockingly bad to the tune of 600 shots thats two dives in my book, and one less battery change than needs be.
Would I be right to say that the D800 can flash sync to 1/320 where the D850 and D500 can't and will only go to 1/250.?
Other than that then its a choice of the two, if you've always had cropped then stay as you are.
You cannot compare image quality on any camera and say its the camera, as optically you will always get a better more optically pleasing image with a prime lens as to a zoom lens say 16-35mm.
To compare the full frame 16-35mm to a prime lens is just simply wrong as the two will not stand up together. I own both for the 800 and the 15mm fish eye is the tits compared to the 16-35mm zoom lens, but then I'm using that as more of a portaight lens under water, but this is about the two cameras and not about the lenses we all use underwater..
I said all that without swearing.!!!