Pro One is brighter with the filter open, has better bass, waaaay punchier envelopes, additional bus for modulation routing using modwheel for depth control. cross-mod and audio-rate PWM, the ability to combine square+saw+triangle LFO and VCO waveforms, 4 pole VCF, VCF keyboard tracking control, VCF self-oscillation, oscillator sync, an arpeggiator and a sequencer.
CS-15 has multi-mode 2 pole VCF, 2 parallel synthesizer channels(but channel one has only one VCO), separate optional glide envelope for VCO, invertable envelopes, a pitch stick that actually springs back to center and sample+hold LFO.
I got rid of my Pro~One because the filter thins out tremendously with resonance up, the pots are cheap and wear out too fast, the case is flimsy and cheaply made and I kept feeling like the VCO's always had this "stiff" quality about them. I had other monosynths that already covered the other advantages.
Aside from the bass, I find the CS-15 more tonally pleasing. It certainly lacks alot of bells and whistles, and I wish the CS-15 had punchier envelopes, oscillator sync and some kind of modwheel functionality, but just don't use it for bass or spacey sounds and it ought to still be useful.
"Lots of people are nostalgic for analogue. I suspect they're people who never had to work with it." - Brian Eno