If you are considering a D500/D5, might has well throw a D850 into the mix. DX cropped, you are getting something very similar to the D500. Now, you may not get the same high ISO performance, but you'll be getting an FX sensor.
While you can use a D850 in this way, there are a few practical (minor) issues. First, if you shoot with the intention of cropping by at least 1.5x, the DX area in the D850 viewfinder is small compared to that in the D500, which can make it harder to focus and compose on the subject.
Secondly the D850 makes a louder sound than the D500 (in normal viewfinder based shooting). The D850 does have silent shutter mode available but it is live view only, so for this and other reasons, it is not as well suited to action subjects, at least if you need to focus continuously and adjust the composition during the sequence.
Thirdly, the D850 real-life buffer performance leaves something to be desired, though in DX crop mode I guess the issue is not as severe as in FX.
There is one omission in the D500 specifications, namely the 9-point dynamic area mode which is present in the D850 and D5 is absent from the D500. Interestingly the D9 has become my most frequently used AF area mode with the Multi-CAM 20k equipped cameras and I would not be happy if it were taken away. A large part of the module's capability is not available in single point AF area mode (since only a third of the AF points is user selectable) and 25-point dynamic or group area AF do not have the precision of single point or D9.
If shooting in DX crop mode, I would still prefer the larger viewfinder, better buffer, slightly better image quality and quieter operation of the D500. If you intend to make use of the 45MP FX capability (or the other features exclusive to the D850) then of course the situation changes. But 45MP for action can place quite a burden on your storage and post processing workflow. I think while the D850 in theory does everything in practice it is still best used for precision work rather than action.
However, it depends on how you shoot. If you have no need or intention to use CH and if you just want the best image quality at low to intermediate ISO then the D850 may be the best choice. However, if I shoot action I need the camera to respond precisely in time irrespective of what kind of tasks were given to it moments before, and in my experience it is too easy to make the D850 have a hiccup, but this may be partly because I'm used to the D5. I am not a heavy user of CH in fact I mostly shoot in S or Qc modes (in which case there is no issue with the D850), but there are a few applications where I do use high fps, and in those cases the D850 is not ideal. However, perhaps optimizing the settings and use of only the XQD card slot alleviates the issue. I just don't like to have to change settings from situation to situation as works against instinctive operation of the camera. I would like the camera to automatically switch to 12-bit NEF recording when at ISO 400 or higher, for example. This would be a more seamless reduction in file size and optimize the buffer capacity without the user having to remember to change settings. It should be a simple job in a firmware update to permit this kind of grading of file type as a function of ISO setting.
Anyway I am surprised by the apparent great interest in the D850 and relative lack of interest in the D5. I get the cost difference but other than that, even after 20 months with the D5, I'm still happier and more excited about it than the D850.
