Are non stretchy sails getting too much credit again? I think the controllability of sails has come about from mast design/sail relationship. Stretchy mylar is in essence more controllable. However, before active mast tuning, sails stretched fuller in gusts - bad. Now sails can be stretched(less full) with outhaul and mainsheet etc, and mast bend can automatically flatten the sail in gusts (even if the sail stretches the wrong way). The jury is still out on the "benefit" in tunability of stretchless sails.
Mylar film is cheaper than Woven Dacron ....