I looked up these two words in many dictionaries; the only difference I could find was that revealing can be used to describe clothing that overly discloses human body, like a revealing blouse, but in terms of information, secrets or insights..., how could they be differentiated?
With revelatory, the knowledge of that which is revealed is usually beneficial to or of concern to those to whom it was revealed; they have had their eyes opened; they have been enlightened by the revelation:
It was a revelatory experience.
The revelatory memo documented how the company had been dumping toxic waste into the river for decades.
To call the memo revealing would be to say that it had divulged secrets. To call it revelatory would be to focus on the value of those secrets to those to whom the information had been revealed.
When the presidential candidate referred to citizens at the lower end of the economic spectrum as "takers", it was a revealing moment. The electorate certainly found those words revelatory.