Well, if you are talking about one of the 5 original releases, they were the full shows up to that point. There are differences between them, but they are not the "highlights" usually found.
The one big difference is that none of the 1977 versions have the announcement. The music track is there, but no Jack Wagner. It's interesting to hear it without it. So, that means that even if you have the Walt Disney World version, it does not have the intro that everyone wants to find.
So, if I recall, the 5 different versions were as follows:
1973:
Walt Disney World Electrical Water Pageant
Disney's Main Street Electrical Parade (Disneyland only)
1977:
Disneyland Main Street Electrical Parade
Walt Disney World Main Street Electrical Parade
Disney's Main Street Electrical Parade (replaced the above two, so it could be sold at both parks)
Is that right?