I made the switch, which was a bit of gas though rather than need. I wanted a D810 when I bought my previous three D750s but the price was just out of my range then.
What I prefer, the cropping ability, quieter shutter a lot better than the D750, you often mention the grip but I see no difference from the D750 and find it comfortable. For my bird photos I often run in 1.2 crop mode, so it's like having another camera. You can black out the outer crop mode which makes it easier for framing. I feel the focusing is better when using the group area AF over the D750, especially for BIF. Never used the flippy screen on the D750 so not a issue for me, if I wanted it for low work, I'd just use my tripod in macro mode.
Colour wise I find them about the same but the JPEG's are better from the D810, I remember when I first had the D750 how poor the JPEG's were compared to the XT1 I moved from. It's in the early part of this thread. Finally I find the the camera sharper as expected and for me the blacks seems to come out a lot better. As for Noise, I was a little disappointed at first but, looking back I cant see much of a difference over the D750 files, it was in my head.
. You also gain a bit with the D810 with the ISO64 and the 1/8000 shutter speed.
In summary, I would be more than happy with a D750, but I do prefer my D810. I did have it up for sale due to some medical issues, but I have just bought a GX80 as a lightweight option and will keep the D810 for home use for the time being.
Hire one for a week and see how you feel, rather than reading IR, DXO and DPreview etc.....
Just a record shot and my first Kingfisher photo, but to give an idea of the cropping capabilities. Bird was about 100+ meteres away.
Original photo of KF, see the crop on next photos. by
Swansea Jack, on Flickr
1st Kingfisher shot, very large crop though. by
Swansea Jack, on Flickr