I'm doing similar calculus myself. I shot a D300 for a long time and 'sidegraded' to a D7100. The D7100 was an upgrade in some areas and downgrade in others.
I am limiting myself to the D810 or D750. And until the D750 is fully exercised it is hard to tell if the specs actually indicate performance. My bottom line is if the D750 feature set was packaged in the D810 body, that is what I would buy!
Looking at what I feel are the biggest differences:
D810 button interface versus D750 dial interface - I can tell you that the D300 was orders of magnitude less frustrating to operate than the D7100. Example, I was shooting on the coast at morning twilight and needed to switch from to M mode. On the D300 it would have been push button and spin wheel while looking in the viewfinder. On the D7100 it was pull out the phone, shine the light on the Dial on the top of the camera. Try to hold the phone and push the little button in the middle while using the other hand to turn the dial! Thank god it was all mounted on a tripod! Why Nikon would perpetuate that half assed, 1970s interface and segregate their lower end consumers from a clear upgrade path to more expensive stuff is beyond me. (And don't get me started on why neither the consumer or pro side has full nameable shooting banks when every setting on a modern camera is set electronically!)
Size and Weight - I actually prefer beefier feeling cameras. I am steadier when hand holding. The D810 is much more comfortable in my hands even though I have small hands. If you the weight and size are constraints, then obviously smaller lighter is better.
36MP versus 24MP - I actually don't need 36 MP for most of what I do, but if you are doing things like Multi-Shot images where part of the image will be 'wasted' in the stitching process or if you can't afford the long lenses and do a lot of cropping, then 36MP might be handy. The D810 in DX mode gives a little faster frame rate and a few more MPs than the D750. I intend to keep the D7100 so not expecting this to be an issue for me. Also, the D810 pixel size is 4.87μm while the D750 pixel size is 5.95μm at the same sensor size. That means the D810 is exhibit more diffraction blur at the same apertures. I've attached a JPEG showing the 1x, 2x and 3x diffraction limits of the cameras.
Low Pass Filter - Theoretically, the D810 should exhibit a little more per pixel sharpness at the expense of potential moire and diffraction.
Noise and High ISO performance - Theoretically, the D750 should exhibit less noise and better high ISO performance just based on the larger pixel size, but real world performance has not yet been confirmed.
Dynamic Range - This should be a horse race, but again not verified.
AF Performance - Theoretically the specs say the D750 should be better, but just because it uses a new CAM module doesn't mean that module is better. I'm not expecting a big difference.
WIFI - Having this built in with the D750 could be especially handy if you use remote control shooting like Helicon Remote. Not sure I would do much sending images to my phone and such.
Tilting Screen - If you shoot low to the ground or in other awkward situation, this could be handy.
***UPDATE - Usability***
The D750 has a square viewfinder window and D810 round meaning various accessories MIGHT fit. I have an old Minolta Right Angle Finder that was modified to fit my Nikon Film bodies. It might work on the D810. The D750 has a higher eyepoint though.
The D810 has a viewfinder shutter while the D750 requires the use of the DK-5 plastic window guard when doing longer exposures.
The D810 has the 10pin connector for hooking up various Remote Releases. My ML-20 is a lot more functional than the MC-DC2 that is used on my D7100.
The D810 has both AF/AE lock and AF ON buttons. The D750 does not have a dedicated AF-ON button, but you can reprogram the AF/AE Lock button.
The D750 uses 2 SDHC cards while the D810 has the stupid 1CF/1SDHC configuration. I personally prefer CF for speed and durability, but if I can't get a camera with 2 CF slots, then 2 SDHC slots seems preferable to a mixed configuration. Unfortunately, circumstances may make CF disappear in the not to distant future.
The D810's base 64 ISO combined with slightly higher 1/250th flash sync could be handy.
It is exceedingly clear Nikon has segregated the D800 into the 'pro' camp and the D750 into the 'enthusiast' camp not only by their own designation, but by limiting the functionality and usability of the D750 and designing it to operate differently than the pro cameras. I know several photographers that use the D810 and D7100, but that is only because there is no DX 'pro' body camera since the D300s. I think this is a glaring hole in Nikon's lineup. I'd love to see the D800/D800e disappear and be replaced by a 24MP DX body of the same type as the D810. If fact in the pro range, in addition to the D# full size bodies, I'd love to see 3 identical bodies 24MP DX, 24MP FX and 36MP FX.
