As the other person stated, the one at HS is a short live show with some puppets. It's pretty good at giving you the gist of the whole movie in about 10-15 minutes.
The one in MK is the exact same ride as at California Adventure by Disneyland. Your seashell takes you through the whole story with the best part being the performance of Under The Sea by hundreds of sea creatures. A very good ride, especially for younger kids.