Sunbrella is a much superior product to canvas duck, which is very old school. There is no hard fast rule here, but usually Sunbrella is used for cockpit covers, whereas a coated polyester material, like Auqalon, is used for full covers. Sunbrella is supposed to breathe, whereas the coated material does not, so needs external venting. Hope this helps. Don't hesitate to call or email if you have any further questions.