Some observations on Compressor V2.3 vs Cinemacraft SP2 (WinXP), said to be one of the best software encoders on any platform. I think Compressor compares very well, given the price difference.
http://www.netspeed.com.au/mark/encoders/Site/encoders.html
http://www.netspeed.com.au/mark/encoders/Site/encoders.html