The entry level Nikon and Sony models only use a 12-bit ADC whereas the Canons use 14-bit. As soon as you move away from the native ISO you lose dynamic range. You may not care about this in suburbia but out at a dark site it makes a difference. I figure, why start with fewer stops in the first place?
The read noise isn't all that different between most DSLRs at this point. Older models such as the Canon 1100D hold up surprisingly well in this respect, despite their age.
Differences in QE of 10% aren't that noticeable with a OSC camera out at a dark site. You still need to learn to expose the image correctly. Likewise ISO invariance...it's a bit of a red herring for AP, as if you don't expose sufficiently you're going to have a problem bringing the signal out of the read noise floor. My suggestion to deal with that is to take a shot a couple of stops higher than native gain and look at the histogram, then scale it back to native gain for the real shots.
The thermal dark noise is the great divider for astrophotography. The problem with making objects such as cameras smaller is that there is less opportunity to dissipate the heat generated by the internal electronics when exposing for several minutes time after time. This causes sensor/electronics heat-up which makes the dark noise worse.
I recently did a lot of research on this as I had the (rare) opportunity to upgrade and had to choose between all the great models available now. I went with the Canon 6D, a 4-year old design! I read a couple of reviews comparing the 6D and the Nikon D750 and it wasn't that the D750 was bad, it just didn't do a better job than the Canon. On top of that, the Nikon/Sony models that were theoretically more interesting were way out of my budget. The plus side is that my lenses still fit and there's no change required to my astro software
