I had a Prophet 6 once, and didn't much care for the tone. It sounded far smaller and constrained that other analogs I had around at that time (inc OB-6 which sounded much nicer and more vibrant).
The P6 is a cool synth but it's very vanilla sounding, especially for analog, the VCOs are very hard and tame sounding and the filter isn't amazing.
I sold it, kept the OB-6, but then bought a Prophet 10 rev 4 and.... BINGO! THERE was the tone I'd hoped the 6 would have all along but didn't, not even remotely. 6 was very flat and 2D sounding by comparison to the 'real' Prophet 5 sound in the rev 4.
As someone else said, the tone on Prophet 5/10 rev 4 is sublime, it sounds like it has depth and space (reverb and even chorus at times) with no effects at all. It's all sweet spot, a joy to work with and play, build quality is at least twice as good as P6 esp the bolted on pots and overall feel. I easily forgot about the extra toys on the P6 once I had THAT tone, with 10 voices, poly unision, staked/layered sounds (which still gives a 5 voice super prophet 5 in that mode), the feel of the keybed, and even the LACK of effects and gizmos on boad makes it a much more musical and inspiring source for real synth-work in my productions.
And best of all it takes to effects in the mix (or live) amazingly well, probably because it sounds so good without them you only need a little dose of FX to take it over the edge into pretty much THE best sound you can get in a mass produced modern analog synth (better than OB-X8 by far imo). I'd advise the 10 over the 5 because stacking and poly unison sounds amazing... and then you also have 10 voice option for long runs, smooth chord changes... it's a complete dream synth having 10 voices and addresses my issue with the vintage prophet 5s (not enough voices to do everything I want).
Mono output is IDEAL for me, I have no need to have fancy stereo synth output when I can just record bespoke parts and pan them in the mix, much nicer, and easier to mix than stereo synths... and you can easily modulate more stuff via midi CC or add a sequencer/arpegiator simply enough via MIDI.
Basically imo it's the ideal, best sounding, best looking 'only analog synth' anyone could ever choose in modern times and it can do pretty much everything and do it well (other than bandpass sounds), esp with MIDI modulation and after-FX/Filters in the mix to 'fake' bandpass or whatever... I must stress again the sound quality of the base tone is that good that it can be manipulated further down the chain while still sound better than lesser synths that have it all built it and are less sound-mangled later.