FWIW, here"s a piece that I wrote for a newsletter a year or so ago. All opinions are my own, so feel free to disagree with them. BTW, I couldn"t afford to keep them both, so I kept the Prophet 6, and reluctantly sent the OB6 and (not so reluctantly) the DM12 back. You can"t really go wrong with either of the DSI"s, but I felt that the Behringer, while fine on its own, suffered in comparison with the other two quality wise.
DSI Prophet Sequential 6
Very expensive (£2361 for synth, £1950 for module). No splits or layers, and only 49 keys, but the sound is out of this world!!! Very good key bed too. The module (21.25 x 7.43 inches) would be my choice, rather than the version with keys, it takes up a lot less space, and I have recently come round to the view that modules paired with a top class midi controller is probably the way to go.
HOWEVER, Behringer do the DeepMind 12 which is around £560 for the module version (£640 with keys), and it is nearly comparable! And you can get lots more sounds out of the DM12 because of the greater number of effects and the more sophisticated mod matrix that"s on it. Granted, the Prophet has knobs instead of sliders, and sounds a fair bit better, and has a far superior key bed, but the price difference is unbelievable. The DM12 has a screen, which gives you more info, but also leads to more menu diving, so it"s a two edged sword, the Prophet is faster to use. If they were the same price, it would be no contest. At 50% more, I would still go for the P6. But at three and a half times the price, when the DM12 is better in some ways, you have to think very very hard. Why do DSI charge so much? Because they can!
OB6.
The Tom Oberheim version of the Prophet 6, with the same astronomical price tag. The advantages are a huge bass tone that is so phat that it can be a touch overwhelming sometimes, and a notch filter setting, which sounds amazing, I mean, it"s really out of this world, far and away the best that I"ve ever heard. Apart from these two (not inconsiderable) advantages, the Prophet 6 is crisper, clearer, more versatile, and better in most ways, notwithstanding the Oberheim voicing. It is very easy to make the OB6 sound 'fizzy' on some settings. I know that at least 50% of people would disagree with me, but I know which one I think sounds best. I can"t help wishing that the P6 had the notch filter though!