Hi Helen~
I'm an ACS instructor and have used both products. I think you may have a bad batch -- slow dry usually doesn't start to dry in seconds. I recently got some clay that did exactly what you're describing. It is frustrating.
If it ever happens again, get some Cling Wrap (or Saran Wrap), put the piece on the plastic, add a drop of water, completely cover the clay, and wait about 10 minutes. That usually puts enough moisture in to keep it from turning into alligator skin. The good news, of course, is that you were able to at least turn it into paste!
Sorry you had such a lousy first experience. There are good and bad points to both PMC and Art Clay, and it's good to be able to use both types.
Good luck, and let us know what happens in future experiments.