When we started Hyperflite, our goal was to design a disc that had an easy shape for dogs to catch, would fly and perform well, and be easy for people to grip. After months of rigorous testing and numerous prototypes, we can up with our standard shape. This model, in the regular size, and the Pup size have been incredibly successful and are the discs we recommend for all enthusiasts. Both sizes are available in a number of different formulations (regular, flexible, soft, etc.).
That said, there is a group of enthusiasts who prefer the old-style Fastback Frisbee disc.
The Fastback Frisbee disc was originally designed by Wham-O to be a premium (giveaway) disc. Companies would pay to put their logos on the disc and then give them away. While it was not intended as a dog disc, when the sport of disc dogs began, it was the best suited Frisbee disc available (size, weight, shape) and for many years it was the standard disc used in competition. In fact, Jeff Perry and Peter Bloeme (founders of Hyperflite) won their respective World Championships with them.
So, since there were a number of enthusiasts who preferred the old style disc, we developed the Z-Disc, along with various improvements over the Fastback disc, to address that market.