Some of you may have noticed the dpreview comparing the Sony A7S, A7R and Canon EOS5D. The Canon is quite a bit less expensive. Based on comparing its low light performance using this chart, the A7S is better. There is more detail at ISO 25,600 that would be usable with or without noise reduction. Sony simply wrings more detail out of its pixels.
I'd read this review a while back but it was good to review the results after having used both the A7R and A7S for a time...
Daylight photography doesn't have to worry about the number of photons collected per sensor pixel to generate an image, i.e.: there are lots of them.
Astrophotography and nightscape photography, on the other hand, has to collect enough photons per pixel to actually generate something above the inherent noise of the system to generate an acceptable end result. This normally means longer exposures which is the ONLY way to collect more photons from faint targets. What we are discussing in this comparison is the handling of those few photons.
The comparison shows the A7S and A7R producing about the same level of image quality up to the max. ISO of the A7R (when the A7R image is reduced to 12 MPixels); however, I do note the resolution of the A7R image is considerably better than that off the A7S, and I have found this in practice. As a result I normally use the A7S on lenses/scopes above 500mm focal length and the A7R on wider angled lenses/scopes to retain as much resolution as possible.
The difference between the Sony A7 series and Canon images is entirely due to inherent noise and the differences in handling this, either by "cleaner" sensor technology (pixel size, etc.) or by proprietary hardware/software processing algorithms. At the moment Sony appears to be the winner in this race... How long? Until the next Canon or Nikon technology hits the market?? Or some other camera manufacturer plays a game of technology leapfrog to gain a short term advantage?
Personally I'm not happy with a single high ISO image (sub) off ANY camera! Sure they show something but is it really an "image" worth saving and getting excited about!? Not in my opinion. Now, if you collect a whole bunch more photons to fill in all the blanks/holes in a high ISO image by shooting, aligning and stacking a number of subs (the more the better), thus improving the signal-to-noise ratio, I can see the usefulness of a high ISO, low noise camera. And I have processed stacked astro-images with 30-50x30sec exposures at anywhere up to ISO102400 and they can be quite nice; however, 30sec subs @ ISO6400 (maybe ISO12800) is about all my sky conditions normally allow using an f/8.0 lens/scope before totally blowing out the subs (and considerably less exposure and/or lower ISO with an f/2.0 lens!). 30sec subs at ISO3200 are normally more than adequate (and cleaner from the get-go). And 60sec subs @ ISO1600-3200 are wonderful, not really due to the lower ISO but very much due to collecting double the number of photons per sub; essentially improving the signal-to-noise ratio right at the point of origin, i.e.: the initial sub.
But as I've said (a few times previously), if you're happy with single astro-images off any camera, go for it!