I can respond to both of your comments, because I've obviously got both instruments side-by-side and have many times compared the same patches on each. An eight-voice (or twelve-voice) Poly Evolver is a pad masterpiece. The hybrid aspect obviously allows you to go in one, the other, or both directions at the same time. A number of the digital wave shapes respond beautifully to changes in the filter, making much more than merely typical sweeps, but something more enchanting. It makes superb evolving pads with minimal effects that are both warm and dreamy. That's what I most like about the PEK. But by no means do the two synthesizers stand on equal footing when it comes to simple direct analog tone. In this area, the Prophet '08 wins. If I want an immense brass patch, or a rich string patch, or a sweet sawtooth solo patch, I always go to the P'08.
The Evolver has a cooler analog tone, which is not a problem when you're piling on all the oscillators. There are various ways to compensate for the shortcoming, but when you use a simple clean sound - say, one or two analog sawtooths with minimal effects and modulation - it's quite noticeable.
In addition, the two instruments have differently behaving envelopes. I always use the linear setting on each, yet the response is different. For something like a brass patch, again, the P'08 works much better; it seems easier to control and anticipate its behavior.
The Prophet '08 is also able to make finer increments - be it through after touch or in setting LFO modulation depth. For example, when using the PEK's third envelope for a delayed vibrato, the first increment of modulation depth is already too deep for creating a moderate vibrato.
I realize in one sense the analog side of the PEK is identical to the P'08, but it must be that the AD/DA conversions have a substantial sonic effect. There's no way these two instruments sound the same. If they did, I probably would have sold my P'08s years ago; but, in fact, I much prefer them over the PEKs because I prefer quality over variety.
It probably seems strange to most of you. I have two beautiful PEKs, and one of them is coupled with a PER. And yet, I find myself constantly drawn more to the P'08s, which are significantly smaller and simpler instruments. But that's consistently my ears' preference.