Comparing D810 Pros
- Better grip, deeper similar to D500/750
- Better viewfinder with specs it might seem small but very noticeable and bright
- Focus peaking, yes it works for Live View Photography, you have several colors and thickness of line settings. Only con does not work for 4K Video. No idea why
- Most of my lens are manual focus but tried few times with AF Nikon/Sigma Art lens on the new focus stacking feature. Makes things easier by adding steps. Does not work on MF lenses. Not a con thats the way it is.
- Like the button layout matches D500, slightly different from D810 not a deal breaker for me
- Sensor definitely improved, ISO performance for the same shot from D810 same lighting conditions show much more micro contrast in images with great noise control. Note not a big change in base ISO 64 to 800 from D810. D810 still great
- Shadow recovery, seems to be better so better Dynamic Range when compared to D810. Almost close to GFX 50S Medium Format camera but in 3D pop GFX wins
- Speed is good, I shoot mostly landscape so its not a big deal to me. I use SD UHS-ii 300MB/S card.
- Tilt screen is awesome addition for low shootings, only complain here is I wish it flips in portrait mode like GFX 50S
- Battery life is awesome

Now Cons:
- Would have been nice to have focus peaking for 4K, maybe firmware upgrade Nikon?
- Missing built in flash not that I complain much but that one odd dark place without your other flashes D810 it came in handy. So keep a flash in bag
- Wish the battery stayed the same as D810 and comes with bigger charger like 2 at a time as base
- shutter speed beyond 30s is still not possible without remote

If you have lot of cash lying around and you're a pro would make it back definitely buy. If you are with D800 yes this is a worthy upgrade. If you have D810 its 50/50 upgrade not a big difference but there are features like 4K video, focus stacking, focus peaking (for me this is useful with my MF lenses).

Happy shooting and may the light be with you all!