Chopped strand mat consists of short strands of glass held together with glue, called a binder. It is very economical and offers excellent stiffness. When the polyester resin dissolves the binder, it frees the fibers allowing it to move independently. As a result, the mat can move in any direction, conforming to complex shapes. Epoxy does not dissolve the binder, so the mat stays relatively stiff. You can use it, but it will be harder to work with compared to polyester resin.