Going from a D3100 to a D7100 would be a step backwards in terms of astro imaging:
http://www.dpreview.com/reviews/stud...64.acr&x=0&y=0
As you can see noise performance at high ISO is quite a bit worse on the D7100. D7200 would be cleaner. To get a decent gain you'd be better off getting an older but cheapish Canon 5D or get a 350D that had been modified.
In the APSc camera world the cleanest camera I am aware of is the Fuji X series. XM1 (cheapest at $793 body only), XE1 (has an electronic viewfinder and about $815 body only). Sony Nex cameras like 5N and 6 are fairly clean as well but noisier than the Fuji. Basically a Fuji X or a Sony Nex generally speaking tend to outperform their APSc cousins from Canon or Nikon but any are capable of good images with the right exposure strategy.
Canon, Nikon latest offerings would be next best. Nikon used to be bad for astrophotography because the hot pixel suppression algorithim used on RAWs was too crude and confused faint stars with hot pixels and would damage them. This has been corrected in later models although I don't know at what point it was improved. This gave Canon a lead plus the very early Canon 10D and Nikon D70 the Canon was worlds ahead of the Nikon. The Nikon gave bad amp glow on long exposures, had the difficulty with the hot pixels suppression damaging stars so Canon took the lead. In a way Nikon last the race at the start and so the industry seemed to form around the Canon models.
A lot has changed since then and I think the differences are minor with Nikon having an advantage in the fact they choose the best sensors from several sensor makers as opposed to having only what Canon makes themselves even if they are good at it (now getting dated and needing some updated fabrication plants that run at the smaller sizes Sony etc do. But the top cameras like Canon 6D 5D3 Nikon D600 and D800E all take similar images with minor differences.
QHY has a range of affordable intro CCD cameras. One with a smallish but nice Sony chip makes sense. Sony generally speaking is the new Sensor powerhouse with many camera makers using their sensors and a lot of smartphones using their sensors. They seem to be several generations ahead of their competitors. Pentax, Ricoch, Olympus, Nikon of course Sony, all use Sony sensors. Probably more use them.
MX25C sometimes come up I would think for less than $1000 by now. Look on Astromart.com as well.
Greg.