I own a Prophecy myself and have also played the Z1 and I can tell you that the Z1 has more Physical models although the bowed string model is a really hard job to program any sounds with. The Prophecy sounds a bit indirect as if it were looped through some FX although none has been used.
I think the Z1 has some faster EG's(for better percussive sounds) than the Prophecy.
Last but not least the polyphonic arpeggiator of the Z1 is a real great deal. It's more like a sequencer.
The Z1 is polyphonic.
But one great plus for the Prophecy is the weird Ribbon- control- wheel that is great for manipulating more than one thing at a time.

P.S.: The Z1 Factory sounds are real bad, but it's capability if you do the progamming job on your own is great!

Yours

MCML