My two cents worth is that it isn't reasonable to compare the D500, which is a relatively new camera vs the D800, which is now 4?, 5? years old. Why not wait for just a few months and compare the D500 vs the upcoming D820/D850 or whatever Nikon calls it. It might be fairer to compare the D500 image quality vs the D810 in the DX crop mode. The D800/810 has slightly fewer Mpixels that the D500, but not so many fewer that its theoretical resolution would be much less than about 85% of the D500.
As I see it the D500's primary advantages are the very enhanced auto-focus system and the newer image processor (X-peed 5), both of which are already in the F5 and will almost certainly be in the D8XX. Thom Hogan and others are predicting that the D8XX will be released in only a few more months and will probably be about 46Mp, which means that in the DX crop mode you would get virtually the same Mp as the D500, and have the ability to switch up to the FX mode or the intermediate 1.2 crop mode at will. Even during the dive. This would allow you to shoot the FX crop, the 1.2 crop, and the DX crop all on the same dive, with the same lens, while shooting the same subject.
I recently bought one of the new Nikon 8-15mm fisheye zoom lenses. In the past I have briefly shot a few topside shots with three different Tokina 10-17mm lenses. Everything I shot with the Tokina zooms was so lacking in image quality that I felt shooting the Sigma 15mm f2.8 fisheye or the Nikon 16mm f2.8 fisheye and cropping the image to equal the coverage of any Tokina shot would result in better image quality. Now that the Nikon 8-15mm lens has arrived and covers a very similar zoom range to the Tokina, but with much better image quality. By switching a Nikon FX camera from DX crop at 10mm, to the 1.2 crop and about 12mm, and then 15mm at the FX crop, you can get a stepped variation of the zoom range with progressively higher and higher image quality. Or just shoot everything in the FX mode at 15mm and crop each image to suit.
I am looking forward to seeing and shooting with the D8?? when it arrives and will be perfectly satisfied with my D800's until it does.