I don't have my two sets in front of me to give a direct card-by-card comparison (I can do that later if desired), but here's roughly the differences between the two sets.
The original edition of CotDP has 'Barred from X' cards, where X is a neighborhood. An Investigator gets one of those, they can't go into the relevant neighborhood until the terror level increases and the Barred card goes away. The revised edition replaced them with 'Patrol' markers. When a patrol marker is in a neighborhood, all investigators that pass through a patrolled street must sneak through or be arrested. Similarly, Patrol markers go away when the terror level increases.
A new mechanic to the expansion is the 'Whispering Exhibit' encounters. There's a marker on the board in a street area. If an investigator is in that street area during encounters, they have a Whispering Exhibit encounter. This usually gives the Investigator an opportunity to get an Exhibit Item by passing some sort of check. Then, pass or fail, the encounter specifies a street area to move the Whispering Exhibit marker to. If no one has a Whispering Exhibit encounter during a turn, the marker moves as a moon monster.
There are also a significant number of card changes. Most, if not all, of the benefit/detriment cards have been re-written to provide different effects. Some Mythos and encounter cards have been rewritten in a effort to be clearer. A good portion of the Exhibit Items effects have been changed, especially the Masks. Most of the Allies have been reworked as well, they no longer provide +2 to 2 different combat states. The Messenger now gives a -1 to everything! Some spells have been changed as well. Plague of Locusts now returns a single monster to the cup rather than weaken everything in the neighborhood.
The cards in the Miskatonic expansion that are meant for the revised edition are additional Whispering Exhibit encounter cards.
I'm curious though, how recent is quite recently? As the revised edition has been out for just over half a year now and there wasn't an additional print run of the original edition for a decent period before that, I'm surprised you found somewhere that was still selling it.
As to whether or not the expansion is worth purchasing, I think the revised edition is better than the original. One of my (and many other people's) biggest complaints about the AH expansions is that certain mechanics get severely diluted once you mix in more than one expansion. With the Original edition, as you added in more expansions, the odds you would see an encounter that would mention an exhibit item (let alone actually getting one out of it) would plummet. The Whispering Exhibit mechanic rounds that out nicely; They're always available. Personally, I also thought that the allies in the Original edition of the game were too strong and was glad to see them toned down. The Barred from neighborhood cards were also rarely important. It was rare that one would show up, and it only affected a single investigator. Patrol markers still have to be triggered by encounters, sure, but they only create a minor obstacle for the entire team rather than completely preventing someone being useful in a given neighborhood.
