I can't speak on behalf of the Mk.2, but I briefly had a DD Mk.1 in my possession. It was poorly constructed, the customer service was pretty much nonexistent, and the effects module was extremely noisy. The only thing that stood out to me as special was the HPF, which remains the only HPF I've come across that I'd use more than just rare occasions (mainly to take out muddy low-end). Other than that, to me there's nothing remarkable about the CS-80 sound. It was more about the expressiveness with the large throw keys with polyphonic aftertouch and the ribbon controller.
As for the P6 vs OB-6, it really is as everyone says. If you're looking for character, then go with the OB-6, but if you're looking for overall class go with the P6. You'll spend a lot of time trying to tame the character of the OB-6 if you're merely looking to replicate other synths.