The ISO can be dialled in using one hand on the D810 as well, by reprogramming the function of the Video button.
True, my video button is programmed as the ISO button.
But, again, it's more than that. The AF +
the reach of the D500 make it the better camera for wildlife.
Another element is also the LCD ... it is terrible in the D810 (terrible for high-mag macros, dark, grainy, unless I am in fairly bright light--or use artificial light).
The LCD in the D500 is very nice, much brighter, much clearer, and the touch screen is wonderful.
Again, I have both camera, and I bring both cameras, but for wildlife
the extra reach +
the much better AF mean the D810 hangs on my hip, in a camera holster, used infrequently, while my D500 sports the expensive action glass and is what is always in-hand.
I don't even use my D810 + Voigtlander for nature macros much anymore. (Why attempt to get super-close to a butterfly, when I can get essentially the same-quality shot
from 10' away with a D500 + 300mm + extender?)
I only use my D810 if I (1) have optimal light, (2) am able to shoot a non-mobile subject, (3) can use base (or near-base) ISO, and (3) use a tripod to get an ultra-clear image with a lot of DR.
Under those perfect conditions, I love my D810
However, if I am trying to capture "a fleeting moment," where I spot some wary, mobile creature in the woods, the D500
is the action camera--and it excels at its job.