Well, for prop purposes, unless you're making something huuuuge like a Proton Pack and need the extra strength afforded by fiberglass-inlay, stay far away from polyester because it stinks like hell. Polyurethane can be poured (in smaller quantities) in a well-ventilated hobby room. (Some do give off less-than-healthy fumes though- even if they are odorless, so use a mask anyway and keep it well ventilated!)