In all “objectivity”, the Prophet sounds punchier, while the OB sounds creamier.
Speaking of other little differences.
* The Prophet 10 desktop gets considerably warmer during operation than the OB-X8 desktop.
* The Prophet is easier to program thanks to a knob per function layout, while OB-X8’s way of setting up multis (double, stack, etc.) is practically unmatched, even when compared with many modern workstation synths.
* The Prophet has an A440 reference tone, while the OB-X8 amazingly does not.
And other little similarities.
* OB-X8’s calibration takes a similar amount of time than the Prophet 10’s. Naturally, OB has a shorter “tune”.
* They both sound and look gorgeous.