It very well could be polyurethane vs. fibreglass. This is all relatively new to me. I have only worked in theatrical type paper amche and clothe pieces. But I do have to say the tutorials and discussions in here are great.
Polyester resin is the resin for fiberglassing and polyurethane resin is casting resin. Fiberglass is just the mat or cloth you use for the reinforcing. You can use polyurethane and fiberglass mat or cloth together as well.