first up had the 5100 for over 4 years now, not a bad body but wanted a upgrade also wanted something weather sealed and a bit more robust.
on what i shoot alone im probably more leaning to the d500, but was looking at a d750 for the better low light capability. this is why i was after some general thoughts on if anyone had used both.
What is it about low-light that you particularly need? For landscapes you need a tripod and the sensor has little to do with it. When indoors you probably will be need to uses flashes even on a FF body. If it has to be hand held and natural light then I assume you have budgeted for a set of f/1.4 primes? Otherwise you might just be better served by a crop sensor and some modestly priced f/1.8 primes. Or just a good tripod.
The low light capabilities of a FF sensor are mostly in regards to shooting sports or wildlife where you need a fast shutter speed but a telephoto lens with reach, so you are lucky to be at f/2.8 and may well be at f/4.0 Then you need all the help you can get.