Both types of shower pan use fiberglass as a backing material. The difference is in the top layer. Fiberglass pans have a relatively thin (1/64th inch) sprayed on polyester resin “Gel-Coat.” The acrylic pans begin with a thicker (1/8 inch) plastic layer that is molded to shape and then built-up with fiberglass…or the whole thickness of the pan may be molded from acrylic. In any case, the benefits of the acrylic pan are better durability and scratch resistance (and scratches don’t show because of the solid one-color full thickness of the material). The downside is that a complete breech of the membrane isrepair than fiberglass (fiberglass patch kits are available at your local boating supply store), and, of course, the cost).