The Nikon D750 is a full frame camera that doesn't sacrifice the extra design features normally found only on more entry level options. As the first full frame DSLR to offer both a tilting LCD screen and wi-fi, the D750 offers quite a few design and usability perks. But besides just the extra design features, the D750 sensor scored pretty high in testing, despite having a bit smaller resolution than the D810. That lower resolution means the camera can process the files slightly faster than the D810 as well, offering a 6.5 fps burst speed.
While the D750 includes an excellent design and pretty solid image quality, there's a few things to be aware of. First, the flash sync speed is a bit lower than most at 1/200 instead of 1/250. The maximum shutter speed is also a bit lower at 1/4000, where most head up to 1/8000. Besides just having a lower resolution than the D810, the D750 also still has the optical low pass filter in place--depending on how you look at it, that can be a pro or a con.
The D750 is the only full frame option to include a tilting LCD screen and wi-fi in the same body. That extra feature sacrifices a bit in terms of resolution compared to similar models. The D810 has a higher resolution and also removes the optical low pass filter, though it's a few hundred dollars more. The Canon 5DS, which doesn't have that tilt screen, offers a 50 megapixel sensor but doesn't sit close to the D750's price point. While the D750 looks like an excellent camera, photographers need to consider the higher resolution and faster flash sync speed on the D810 before making the final choice.