In my view each is very importance and plays a part in the final strength and toughness. The 13 oz polyester material supplied by George Dyson is made for use as a filtering media. As such it is very closely woven with a high thread count. An examination of the looser lower thread count nylon material I have seen used on boats compared to this 13 oz polyester shows this great difference. 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.