I'm going through the same process and have decided on a pre-loved Nikon D600/D610 which you should get for your budget. The extra real estate offered by the full frame Sony sensor, which it shares with the D750, is a huge plus for wide-field shots, particularly of the Milky Way.
My thinking is that the bigger pixel size of the full frame sensor offers much higher light grabbing ability than the smaller pixels on the cropped sensor, and that is what this genre is all about, gathering dim light.
In a different scenario, like shooting birds in good light, the higher pixel density of the crop sensor
may offer more detail.
Have a look here
http://www.digicamdb.com/compare/can...vs-nikon_d600/ for a detailed comparison on the two camera's sensors.
Good luck with your deliberations.