I have not played either, but through watching videos of the two, they do seem remarkably similar. If I had to choose between the two, I would take the Spirit, not because of its rare status, but because it was co-designed by Bob Moog. His employment with Crumar is a cool bit of history that I'd like to own, and I find the Spirit to be visually appealing. Can't say that about the M/P. A lot of the gear Korg makes is ugly to me. A lot of people say that an instrument's appearance doesn't matter, that it's the sound that truly makes a good instrument. I don't agree. I think an instrument's appearance can alter a musician's perception of his tool, and can thus alter the outcome of any piece of music he composes with said tool. I like to be visually stimulated by the instrument I play.
Comparison is the thief of joy.