[gregbradley]

Most popular is the Canon 6D. Next most was Canon 5D 11 but some have upgraded to 6D from that. There are also some Canon 7D images (7D is noisy). Canon APSc models 70D down to 40D. But I think most are a bit noisy. You can see at dpreview.com the various models at high ISO in RAW (not jpeg as in camera noise reduction will be in play and confuse results, nightscapes are usually shot in RAW).

On the Nikon side there are some D800 and D800E. The occassional D7100 or D5100.

I sometimes use a Fuji XE1 which is about the cleanest in the APSc sized cameras and is close to full frame performance but not quite.

The new Sony A7 and A7r would be among the best.

For nightscapes I think your choice is determined by low light high ISO performance. DPreview.com has tools that show comparisons of various cameras at various ISO levels.

Canon:
1DX 1DC most expensive pro models
6D (the cleanest of the Canon's probably achieved by firmware dark subtract work rather than a high performing sensor but regardless it is very clean) about $2000
5D111 About $3,500
5D 11 (a bit noisy by the latest models standards) 2nd hand about $1500

Nikon:

D4 most expensive pro model
D800 D800E about $3000
D610 about $2000 competitor to Canon 6D. Oil/dust issue from shutter onto sensor is claimed to be fixed with this model from D600.
D7100 D5300 all very low noise at high ISO and a lot of the Nikon models now do not have an antialiasing filter which makes them very sharp. About $1400 to under $1000.

Fuji:
XPro 1 about $1800
XE2 about $1400
XM1 about $1000
XA1 (cheapest) about $700

Sony:
A7R A7 (these most likely will be the class leaders in sensor being same as D800 and D610 but the 36mp one has been tweaked plus Sony has some in camera selective noise reduction and tweaked microlenses). A& about $1600 A7r about $2400 body only.
Both will have kick ass low light performance equal or better than the Nikon D610/D800/E for less money, smaller, less weight and latest sensor and in camera tools.

Nex 6 Nex 5R Nex 5N (APSc sized) $900 for Nex 6 about $700 for 5R. Nex 6 has same basic sensor as the Fuji X series but Fuji has a different proprietary colour filter array on top of the sensor which results in lower noise. Nex 6 will take shots at ISO6400 but they won't take much processing. ISO3200 is really the max.

Not sure about the alpha cameras A99 should be fine, A77?

Generally Sony makes the best sensors and these are in some of the Nikons (D610 and D800 are same basic sensor as A7 and A7R)
the Fuji X cameras, the Sony Nex cameras, a Pentax camera and possibly some others. Canon makes its own sensors and this is both a plus and a minus for Canon as it falls behind in sensor development presumably by the massive cost involved. Sony is the giant in sensors and sells to other camera makers.

Its a bit early but I would predict the new Sony A7 and A7r would be the king of nightscape cameras. They are small, they are full frame, they have the latest optimised sensors, they have selective noise reduction in camera, the A7r has optimised microlenses to improve light gathering in the sides and corners (maybe only a problem with the A7 camera because the lenses are much closer to the sensor compared to DSLRs).

The lighter/smaller size will make it easier to use on a Vixen Polarie. They use any lens ever made just about (Canon will take a Nikon lens with an adapter, but can't use Canon FD lenses, Nikon will take any Nikon lens made but not Canon lenses.

Sony A7 will also be the cheapest at around $1500 for 24mp full frame, with on sensor autofocus pixels, electronic first curtain shutter (reduces blur from shutter shock), tiltable screen (handy when doing nightscapes), electronic viewfinder (handy for live view magnified focusing at night).

My pick would be the Sony A7 or A7r if you want the 36mp sensor for highest resolution (high resolution is less important for nightscapes, high ISO low noise is more important).

There is a new Nikon about to be released in a retro form. It is rumoured to have the Nikon D4 16mp full frame sensor. If that is true then that is likely to be the best performing low light sensor, even better than D800E. But it might be $3000.

So for full frame (full frame works better than APSc) I'd go Sony A7 or for APSc Fuji XE2 or XM1 or XA1 (cheapest). I'd wait for reviews of the Sony A7 with widefield lenses before I bought one though. It may have magenta cast on the sides problem of Nex 7 with ultrawide lenses. The A7r has specific microlenses to prevent that. I'd wait to see how effective it is. Sony distance from the sensor to the lens is less than half of Nikon and Canon.

The lens you use is almost equally as important.
Nikon 14-24m F2.8 ED is the king ($2000)
Samyang 14mm F2.8 is very good and popular ($500)
Fuji XF14mm F2.8 for Fuji X cameras is almost as good as the Nikon but much cheaper (about $800)
Tokina makes an F2.8 ultra wide zoom that is popular.
I haven't seen any nightscapes examples but ultimate image quality is probably the Zeiss 15mm F2.8 ($3500 lens).
Nikon 28mm F2.8 AIS is cheap and well regarded - manual focus
Samyang 24mm F1.4 is good after F2.
The Canon 17-40mm F4 has some good examples here. F4 is starting to get slow for nightscapes though. F2.8 is what you want.Lenses below F2.8 are probably a waste of extra cost as they usually do not perform well below F2.8 anyway for this sort of work. You'll just get horrible coma or horrible chromatic aberration in the stars (blue/purple halos and comet shaped stars in the corners/sides of the image). F4 and above you are probably looking at longer than 30 seconds on a mini tracking mount like the Vixen Polarie.

Greg.