Well... I found this explanation on the web and it is pretty thorough:

OK, Hobie Cat rudder history lesson:

Rudders were made from ABS plastic. Gets yellow and brittle from UV exposure. Has a bad tendency to snap off right below the casting when loaded up. Spawns a whole cottage industry of aftermarket fiberglass rudders.

Rudders made from Lexan polycarbonate. Will not break, but very flexible. Thin cross section and prone to stalling and ventillation. Not very good for racing.

EPO rudder era - super light, stiff black rudders (epoxy/fiberglass/foam core). Thicker and much more forgiving. Only drawback was the trailing edge needed a lot of work to stop the humming. Still very much sought after as the ultimate racing rudder.

This was an upgrade - stock rudders were still Lexan.

Racer Rudder era - polyester/fiberglass/foam core. Not as light as EPO's, but still a good rudder. Gel coat finish was much softer than the EPO.

Starting in 2004, you could get epoxy/carbon fiber/foam core rudders from Europe. The section was not as thick as the EPO, but they are lighter than the Racer Rudders.

Racer Rudders were also an upgrade. Stock rudders were nylon.

2005 - present
EPO2 Rudder era - based on the original foil shape of the EPO, you have a choice of polyester/fiberglass/foam core or epoxy/carbon/foam core or epoxy/carbon/kevlar/foam core. These are the ultimate Hobie rudder, and at $250 each, they better be. It's nice that they require no prep whatsoever and are well-behaved right out of the box.

The polyester ones are now stock on the 16. For $60, you can upgrade to the carbon one