With a wide variety of fabrics on the market, many factors affect the performance of a cover. Here are some of them.

Outdoor Covers - resistance to year-round weather cycles, including heat/cold/moisture/rain/snow, bird droppings, tree sap (noting areas of dense pines, such as the Pacific Northwest) protection from the sun's UV rays, dust, wetness and/or conditions of prolonged moisture so then too there is mildew and mold resistance. Also it may be important that the cover material be tear-resistance, that it be breathable and provide a smooth light contact with the car finish. Some covers do offer minor protection against dings to some degree. Too, different materials will have different cleaning requirements. And some fabrics require cleaning by commercial methods while others are cleaned as as everyday laundry.

Indoor Covers - resistance to UV rays, dust and moisture. If the cover is for storage -- is it easily moved on and off and stowed?

cover construction: - are the seams single stitched or greater - are the tie-down grommets non-marring - is the material preferred for a neatly fit custom or more of a semi-custom slip on?