I just went through the process of choosing between these synths and want to re-iterate a few points from above.
Firstly, the only real way to pick which one is best is to try the two out for oneself. “Warmer” is pretty subjective; I find the Prophet warmer myself, and plenty analog. But I wouldn’t recommend anyone spend $3,000 based on my choice of adjectives any more than I’d recommend they do so based on someone’s up-thread.
I do however believe that the Prophet 6 and OB-6 are pretty true to VCO-based Sequential Circuits and Oberheim synths respectively. If you prefer classic synths by one brand over the other, you can pretty much guess which one you’ll choose here. My strongest association with Oberheim is the Matrix 6 which was one of my first synths. The OB-6 is a way better synth, but I liked and disliked all the same parts of the OB-6 as I did the Matrix (love the synth choir pads and basses, kind of ambivalent about many of the simpler sounds).
Mind you, I had listened to all the demos and was ready to buy the OB-6, but sitting for 1-2 hours and playing one after the other, I really was far more drawn into the Prophet. For me I was looking for a very solid go-to synth. The Prophet was more like the sort of synth I’d try first when I needed a synth part, while the OB was more the thing I’d look to for certain purposes, or for something a little different.
Again, that’s my taste, and you really can’t go wrong with either. But do whatever you can to play them first, if possible, because it’s the best way to know what’s right for you.