I had the opportunity to test out the D750 and the D810 and the only area of performance I was concerned with was low light capabilities, so I took them out in the dark and shot them both at ISO 5000 and wanted to see what they could do. Now keep in mind I would NEVER shoot at such a high ISO for anything other than to prove a point. Really I hover around 800 if I have too and sometimes I'll push it to 1600 but ISO 5000 is ridiculous and I would never tell you to try to capture anything at that ISO. That being said here's the true test...
The image above is shot with the D750 at ISO 5000 with the 10-24 wide angle at f/2.8 and 1/100s. All I did to this image was adjust the coloring to B&W which should have only amplified the noise level in the image and saved it at 72dpi. (All images should be saved at 72dpi for web and mobile viewing.... remember 300dpi is for print)
This image has virtually no noise at ISO 5000 and a relatively fast shutter speed. (for shooting in the dark) Image is also hand held.... and again no additional digital noise reduction.
This photo was captured at the Triple Rock Social Club in Minneapolis.
The image above was shot with the D810 at ISO 5000 with the 50mm 1.8 at f/1.8 and 1/80s. So... larger aperture for more light (sort of a cheat for the D810 with that lens) and a slower shutter speed which in theory should have allowed for more light to hit my sensor since my subject matter was not moving. BUT.... I reduced the noise in this image in photoshop with 2 filters including Median at 7pt while adding a curves adjustment, 2 color layers, and desaturating a touch to reduce the noise. That's a whopping 5 step process to get one image to compare noise level wise with the D750 image. And I can still see some grain in this shot which leads me to say.... the above photo is garbage, even at 72dpi.
This photo was captured at Donnie Dirks in NE Minneapolis.
What this told me was basically what I already knew.... The D800 and D810 are really for studio professionals that use light to capture images. I don't think natural light photographers would benefit from either camera. Sure they have the most pixels for this range which helps in editing, but I don't know of many "natural light" photographers that shoot for publication so if your business is primarily focused on client work... save your money. You don't need an 8 series Nikon.
The D750 ironically was recalled a few days after I shot the above image - for sensor issues. I wonder if the sensor was TOO AWESOME. So if you want to shoot indoor sports with a f/2.8 or even an f/4 lens this should be your go to camera body. I didn't know if I'd like the flip out screen on the D750 but I used it a couple of times and really liked it as a feature. I don't like to be known as "the photographer" in the room so I try to be as discreet as possible when shooting and well, the flip out screen really allows for that. So I'm anxious to see what the next D750 does that I get my hands on after the recall. If you have any questions about either Nikon body comment below!