It depends! You can use polyester resins with severalfillers I've used glass bead's, cab-o-sil, wood flower, ground pecan shells, and even water . (Wep- Water Extended Polyester). Each has advantages and disadvantages. the biggest issues are the amount of heat generated after catalization, and the dimensional stability of the resulting material. Epoxy compounds are in general stronger. The epoxy is hotter on average.
Rodney Trotter (the uk will get that one) Gandalfs Galloper! Steedfast