The looser weave also absorbs more urethane when it comes to coating the boat increasing weight. Polyester has a very low stretch factor. Nylon is known for its stretch. The combination of thread count, stretch and surface slipperiness gives the feel or hand of the cloth. Nylon is smoother, softer and stretches more than Polyester and as a result it is easier to form around complex surfaces such as a kayak.
Polyester is harder, less flexible and stretches less.
Polyester has a harder hand but will withstand a stronger pull on the sewing thread with out thread pullout. Without this advantage you could not effectively skin a kayak with polyester. You need to get your skin. Ceconite, a polyester material, used on aircraft is available from aircraft suppliers. I have only used one lighter weight polyester material I received from George.