In general terms we say get the best fast glass you can and don’t worry too much about the body, update that later. I think for night photography its possibly the opposite or neutral. Maybe get a good body first, then faster glass later.
Why? Well I’ve been testing a lot of sensors recently for noise, particularly for long exposure noise and it’s fairly easy to see that the cheaper crop bodies do not stack up against the full frame cameras very well at all, at least if you exposing for minutes at a time.
Based on my testing I can’t see a one stop advantage from F4 to F2.8 in glass out weighing the performance gain going from a crop to a full frame body. I have no doubt that a D610 with an F4 lens is going to perform better than a D7100 and a f2.8 or even f1.4 lens.
Of course in the long run faster glass means lower ISO and shorter exposures which for Aurora shooting is important to capture those beams. Just don’t rule out a body upgrade before a glass upgrade. If you have an older crop body camera I don’t think you can expect faster glass to solve all your problems. Consider your upgrade carefully. 🙂
In the long run the ideal is a good body and fast lens, which does not always need to be expensive, considering you really only need a manual lens for night work.