The a7s with its huge pixels blows away any dslr on the market hands down for astrophotography., small high mega pixel sensors are your enemy in this realm. The a7s has the highest signal to noise ration and quantum efficiency of any dslr. The 6d comes in second, I don't know why it handles noise so much better than the other canon dslrs but this camera has proven itself for a long time now, probably also thanks to large pixels. Noise tests and graphs are great but field tests speak louder. When you really get to know astrophotography and can identify the most difficult objects to image in the sky, only then you can truly appreciate what the full frames are capable of. I have seen the a7s expose the ifn, California, Pleiades, love joy, and ifn surrounding the pleiades In a 30s exposure at f2. That is insane. Crop sensors have a very difficult time achieving that type of result. Also it is important to mention that unless you have it modded for full spectrum, you will only be able to image the very brightest of dso's. Without a modded camera it will get boring pretty quick.
Secondly, a full frame requires more expensive filters, field flatteners and reducers, full spectrum mods are more expensive, you have to get a telescope with a light cone that covers a 24mm sensor, and lenses that don't show coma or vignette the sensor if you go that route. There are pros and cons to each camera.
Also, a good crop sensor to go with is the 7dii which comes close to the 6d in performance. But again unless you have it modded you are essentially wasting your time collecting noise on a sensor which is just getting hotter and hotter.