Polyester is also more UV resistant. So even though its often slightly weaker initially it'll maintain its strength longer/better if you use it outdoors.
(Thanks for the thanks guys!)
I still will say though that the sample route (even if you have to pay a few $) is a really good way to get practical knowledge about fabrics...
Another way is to go to outdoors stores and read spec sheets and fondle product...
FWIW most "outdoors fabrics" come in widths from 54"-64", more "home" or residential/clothing fabrics come narrower. (40"-48")
Avoid anything with cotton in it for outdoors "gear" fabrics. Its heavy, prone to holding moisture and won't last.