I have to say my experience with the Tokina 10-17 on a D800 was horrible - and why I didn't even mention it as a possibility. The high megapixel bodies like the D800 and, of course, the D850 show up all the flaws! And as a result I switched to the Sigma 15mm. Yeah, maybe you can use the Tokina at certain settings but I would not recommend it at all on a high megapixel camera. For them, to me, the Sigma 15mm is a no-brainer.
As for the D850 to D750 argument.... tough one. Camera bodies are constantly iterating and, given the cost of charging houses to suit them, it's very difficult to keep up. And to a degree, why would you? There comes a point that if the AF system meets your needs and you pics have the resolution and image quality that you like, why chase a unicorn?
That said, who doesn't like the shiny new toys? Count me in on that one. But they do come at a price and not just the obvious body/housing cost. I think you'll find with the D850 that, as I say, the high pixel count shows the flaws in lenses and you need top quality ones; and then there is the additional issue of managing all those pixels and their huge files: more disc space etc.
Once a camera/housing is a few years past its release date the second hand price drops rapidly. For the housing I'd say it plummets. If you get back 30% of what you paid for it you have done well. So you could sell the D750/housing now but no doubt when the D8xx (or Z8/9!) comes along you'll have the same issue. They are worse than cars for depreciation.
When the D850 was released I went through a massive debate on replacing my D800 - which I really loved. In the end I went back to DX with the D500 and have never regretted it. I find DX easier for wide-angle (back to the Tokina or, as I say, the Nikkor 8-15mm) and it's brilliant too at macro. And the bodies are WAY cheaper. And I don't miss all those pixels (and I do sell my images)
PS: and thanks for the moving wishes. Much appreciated. A big one from Amsterdam to the Caribbean!
