It seems that SBIG gets a good wrap on a number of fronts including price. Customer service wasn't a consideration when first starting this thread - so let's add that to the list of requirements for an OSC.
Given that there is not a huge variance in sensor technology for OSC's in the price range under consideration and it really comes down to 8mp vs 4mp or even 2mp - resolution vs charge capacity (if I understand correctly) - and to what degree that matters given the manner in which the camera will be used. The considerations in no special order for DSLR replacement are;
Charge capacity (QE)
Electronics (noise)
Cooling (noise) - accurate temperature control.
Simplicity (ease of use)
Easy to adapt to camera lenses
Software compatibility
Customer service
Price
Bonus items are;
In-built guiding chip, preferably using the main optics with no requirement for a separate guide scope.
From what I can see SBIG meets those requirements reasonably well.
Any other considerations
that I've missed.
But a list like that may not satisfy purist / specialist requirements.
Alternatively (for my purposes, which are basic), spectrum enhance my 5D MKII and rely on its low noise characteristics as a relatively lower cost alternative. And implement some ideas I have for a sealed, dry air cooling system that retains access to the camera controls.