I have both a Mono/Poly and a Polysix. If I had to keep just one, it would be the Mono/Poly. No question.
The Mono/Poly has:
* Portamento (P6 does not)
* Separate envelopes for VCF and for VCA (P6 has one envelope),
* More waveforms for the LFO (P6 only has triangle)
* The sync/xmod section is bananas...so many awesome sounds in there
* Its speed for the PWM goes slower
* Its arpeggiator is THE BOMB when stepping through the four voices individually (set to different octaves, of course).
In favor of the Polysix...it is polyphonic, if you're into that sort of thing. And, yes, it has patch memory, but this synth isn't very complicated, so I don't usually use it. The P6 also has the chorus/phaser/ensemble effect, but only the ensemble is unique. The chorus/phase wouldn't be necessary if it had 2 oscillators per voice...which it doesn't... it has only one. The P6 also has no velocity sensitivity, which is a bit sad for a poly synth. Finally, it does not have a sustain pedal, which I found surprising for a polysynth.
While it seems like I'm poo-poo'ing on the P6, that's not my intention. I just want you to make sure that you know what you're getting into. While I bad mouth it sometimes, I do like it way more than a Juno 6/60/106. The sound is more alive...maybe it's the VCOs (vs the Juno's DCO) that make it sound better (to me). Or maybe its the SSM filter chips (I love them!). Or maybe it's just the extra amount of circuitry in the P6 signal path that makes it more dirty and lively.
On the downside, you might say that all the extra circuitry makes it sound dark and muddy. It also makes the P6 feel a little less snappy. The envelopes on the P6 are plenty fast, but there's some gain control circuitry later in the signal path that can be slow to open and close...hence, it doesn't feel as fast as the Juno.
With all of these complaints, you might ask why I still have it (and use it regularly). The answer is that the Polysix is pretty easily modified. I've done lots to it...added aftertouch, portamento, controllable detuning, better arp modes, 2-osc per voice mode, sustain pedal, etc. If you're interested, you can check it out at synthhacker.blogspot.com. A good starting place would be: http://synthhacker.blogspot.com/2013/03 ... -demo.html
Even with all of these mods, I'd still keep the Mono/Poly instead of the P6.