I wish it was that simple !! LOL
I have absolutely no doubt that the 750 is a better camera than I would need. The 810 even more so.
But I have been using a D700 for a couple of years, and used a D300 before that, and it comes down to the ergonomics.
To be blunt, the D750/610/7xxx cameras feel like toys in my hands compared to the size and layout of what I'm used to.
I don't mean that they are poorly built, it's just the size, and button layout doesn't feel comfy.
I tried a D7000 but got rid, as I couldn't "gel" with it, for the same reason.
This is why instead of buying the camera, I have been getting my lens lust sorted out.
But now it's crunch time, and I don't need any more glass!
I need to man up and buy one or the other.