Stamoid is a laminated woven polyester fabric
, coated with vinyl - either on one side only (Lightweight Series), or on both (Double-Coated Series). The Double Coated can be heat sealed.
Sunbrella-PLUS-840 is a woven acrylic
with a urethane (waterproofing) treatment on one side.
Stamoid is more waterproof than Sunbrella.
Stamoid is more subject to mildew than Sunbrella.
Stamoid does not breath, whereas Sumbrella does.
I was happy, over 10 years use, with my choice of Sunbrella; although I've known others just as happy with their Stamoid.
I've also known those unhappy with both products, mostly attributable to improper (or no) cleaning