Tuesday I set up my CP4 next to my SP6, and using 2 ZXA1 speakers did a little A-B ing. My overall impressions are:
First, APs: the CP4 adds some overtones to notes when struck harder. The SP6 (using one of Dave Weisers programs) keeps a more even tone when struck harder. I am torn about which I prefer. Both sound great at normal velocities.
Rhodes: the CP4 has a longer decay, and I thought a little richer sound.
Keybed: I honestly kept trying to make the CP4 feel better to me, because of the investment, but the jury is still out on this one. Kurzweil / Medelli has done an amazing job on this keybed, even ignoring its lower weight and price.
B3: I didnt bother with this comparison to the CP4, because I already knew the Kurzweil is in a different (superior) class. However, I happened to have a Nord Stage 2 HA76 nearby and compared their organs. First, control of organs is hugely easier on the Nord. Sound-wise, Id have to give it to the Kurzweil, though. As for keyboard control, the Nord is better; repeatedly playing the same note with rapidly alternating fingers 2 and 3 can be done without a missed note on the Nord, and not so much on the Kurzweil.
So far I have continued to use the CP4 for small group jazz gigs, it have switched over to the Kurz for all others. It may only be psychological, but I think I improv better on the CP4. Time will tell.
Sorry that I didnt do a more scientific test. Perhaps in the future...