To state the obvious, only you know what your musical needs are, so we can only comment on each instrument.
It's hard to conceive of an Evolver Desktop and a Prophet 12 in one thought, since the two are so utterly different. Even putting the monophonic/polyphonic issue aside (and I would expect that would be a main deciding factor), the tonal range of each instrument is very different. The Evolver has much more of a digital character, with its 92+ wave shapes, whereas the P12 offers a much more limited range and struck me as being basically an analog instrument with a small number of digital wave shapes included. Granted, the P12 has other means of producing digital timbres, and I'm simplifying its versatility, but I didn't find a lot of timbral crossover between the two instruments.
The Evolver Desktop, if you can adapt to the matrix panel, is a remarkably complex and unique-sounding mono synth with a broad stereo sound. It's impossible to adequately compare it to a monstrous poly synth like the P12. But, in terms of timbres, I would say the Rev2 and P12 have much more in common than the Rev2 and the Evolver.
On the mystery of the Evolver's missing rubber buttons, I wonder if that might be a reference to the small square rubber feet on the bottom of the instrument. They're only glued on, and these definitely fall off in time, as have the feet on my Prophet '08.