I've shot twice with a D750 and I have no idea where I manage to save the files...
both ISOs are outstanding. you'd be only noticing some major difference above ISO 6400. from 1600 to 5000 my D500 was giving more sharpness, D750 looked kinda soft. above 6400 you will notice more grain in the D500. I prefer the DR from my D500.
low light AF on the D500 is solid, D750 can only compare when using the assist light.
what are you using your D750 to?