Fred Coulter
Collector of ancient keyboards
The PX-860 has 18 preset tones and 0 user tones; the PX-560 has 650 preset tones and 400 user tones.
Going back to the polyphony question, the 256 note polyphony of the PX-560 would be much more useful than the same polyphony on the 860. Unlike the 860, the 560 can access those preset tones and user tones on all 16 MIDI channels. This means that you can use the 560 to play a full recorded arrangement of a piece. And THAT'S when polyphony matters.
So with the 560, you've got a good electric piano that can do so much more (similar to my DGX-650). You just have to decide if the extra cost matches the value of the extra capabilities. And that's something only you can decide.