Look at the different materials as well...as has been stated, you get what you pay for!
I had the factory Sharkskin that was13 years old...I almost always tow long distance with it on as encountering heavy rain while towning a boat, sucks...water gets everywhere! It's still in decent shape; however, I decided to replace before winter as our snow and winds here are brutal.
I just ordered the Westland custom fitted, Sunbrella cover and it matches the quality and fit of the factory original (I'm like 95% positive Westland provides the factory covers for Cobalt as the design and what not look identical?). The new cover has a 12 year warranty (2 years, free replacement and then prorated over the remaining 10 years). Quotes from $565-$1,200...