I’ve never done a side by side comparision but I’ve found both programs will usually give you good results. Compressor’s intergration with FCP is nice but it ties up the program. Compressor does a good job of automatically picking up the settings of your sequence from FCP and setting things accordingly. For Sorenson you need to add the extra step of making a QT reference movie but that’s really not much of a hassle. You might also have to set the settings manually more often.
One thing I’ve found about compressor is it’s 2-pass mpeg2 processing is painfully slow at times as are the frame control rendering times.
Give Sorenson a try and see how it works for you. Since you’ve got it already it can’t do any harm.