I just recently bought the OB-6 and got a chance to play the P6. I'd say they're both as good as it gets to my ears. I certainly wouldn't mind owning both, yet don't feel any great need to.
Prophet is (Peter Gabriel, Phil Collins, Tears For Fears) while Oberheim is (Van Halen, Bob Jovi, Prince).
The weird thing is I listen to those Prophet bands and not the Oberheim ones, but still went with the OB-6 for some reason...
While that is certainly, generally true, I wouldn't pigeon-hole the Oberheim sound too quickly. A truncated list of artists recording with the OB-8 or OB-X/XA would include:
Art of Noise, Depeche Mode, Prince, Simple Minds, The Police, KLF, Soul II Soul, Pet Shop Boys, Van Halen, Thompson Twins, Christopher Cross, Earth, Wind & Fire, Eurythmics, Electric Light Orchestra, Bruce Hornsby & The Range, Japan, Billy Joel, Chaka Khan, Jean-Michel Jarre, Kool and the Gang, Madonna, Nena, Olivia Newton-John, Robert Palmer, Queen, Rush, Styx, Tangerine Dream, Yellowjackets, Miles Davis, Stevie Nicks, Rod Stewart
... and I'm quite certain the Prophet 5 / 10 / 600 list would be as impressive. And both lists would have considerable overlap. They are both quite versatile synths.