I am in the same upgrade process but from a D7000.
I was between the FX D750 or the D7200, and then came the D500 and included it as a third option, mostly when I came to the conclusion (after reading a lot of articles and debates in this forum) that in my case is better to stay with DX and continue to take advantage of combination of small domes and tokina 10-17 versatility, deep of field and increased range for macro, that cropped sensor gives.
I do uwphoto since many years and use to publish my work in magazines, always as a hobby (because my income is not related with uwphoto or diving). Because of that my diving is limited to one or two dive trips a year (unfortunatly).
According to this (and to my budget), money is an important factor in my photo decissions and D750 and D500 bodies double the price of the D7200.
In terms of housing, I use Seacam and there is a significant diference in cost between the Prelude and the Silver line. For the D7200/7100 and D750, you have Prelude models, and for the D500 I asume that will be a Silver housing.
This means that altought the difference with the 750 will be just the price of the body, in the case of the D500, the combo housing-camera will be about 3 times more than the D7200 option.
On the other hand, if you need a second body as a back up, you can get two 7200 for the cost of one D750 or D500, beside the fact that you can also use a D7100 as a back up for less money!
So the following question is: The quality of the images I can get with these systems justifies (insist, in my case) the bigger investment?
I have no doubts that the D500 is a better camera than the D7200, and also that the D750 could perform better in terms of image quality because it has a full frame sensor.
But after reading many reviews of cameras and DX-FX discussions, most of them here in wetpixel, I couldn't arrive to the absolute conclusion that the quality of the pictures I can get with a D7200 will be clearly inferior than the ones I can obtain with a D750 or even with the new D500.
In fact, something I read in posts and reviews by recognized photographers is that many of the major advantages that those cameras certainly have above the D7200, are more In the paper than in the real world of the common photographer, and therefore will not have a conclusive impact, at least in the kind of underwater photography I can do.