I have seen earlier image samples comparing D810 and D7200 with different low dollar lens setups to reach 600 mm. It's essentially a toss-up, possibly with a slight edge to the D810.
I just checked the comparisons in your D7200 review. I gather that the first shots are D7200 and the second D810 and that you used the 300 mm lens on both.
In that case I give the edge to the D7200.
The D7200 does well in the comparison but it has the advantage of down-sampling to the resolution used while I had to crop the D810 shots to get to the same size. If I start to crop the D7200 more, it starts to lose some quality but it can still be cropped while I was almost at the max with the D810. When compared to a 24MP sensor, the D7200 should even do better.
As long as I can fill most of the frame, the D810 is the better choice.