Polyester resin boards are built over Clark foam. It's the original cheap way to make surfboards. The resin is weaker than epoxy resin. The fiberglass used is the same for both types. The foam cores used with epoxy boards are usually stronger. Hard PVC type foams, but some production boards built in China have styrofoam and hard PVC inside them. Boards from RRD are like that. They break.
Poly boards dent real easy. They break easy too. I would not buy one. If you do, don't land it flat and hard. They are easy to repair with sun curing surfboard repair kits.