D750 is a good gear: Full frame, flip screen, excellent focusing system; lots of features that enabled me to do my job. I am most impressed by her excellent focusing system, her 3D tracking AF can stick to my son who is running unpredictably, also it does well for shooting birds.
Of course D750 is not perfect, no camera is. After using the D810, I find myself attracted by her beautiful image quality. With the flagship sensor, D810 gives significantly more detail and dynamic range when compared to her younger brother D750.
When taking landscape photo, D810 is the clear winner.
It is a bit heavier than D750, with no built-in wifi and flip screen, obviously, this camera is not designed for sport photographer. I am not saying D810 is incapable to handle fast moving shooting; her AF system is fast enough but her low fps (5fps) is an achilles heel. One more thing, D810 shares much larger buffer size than D750, although the later one has higher fps, D750 will fill up her buffer very quickly.
Well I enjoy both of them.