Unless you purchase a custom-fit cover that uses some expensive, exotic material, IMO there's not a lot of difference in longevity from one brand of cover to another.
The weather in your area (UV, wind, rain, etc.) and how well it is installed (and fits) will determine how long a cover will last. The high UV's you find in many desert SW locales will turn most any cover to dust in 12-18 months. Wind will destroy a brand new cover in one day if the cover is not installed properly.
When we covered our 33 ft. TT, it tooks us about 4 hours to prep the TT before we ever installed the cover. We applied painter's tape at every wear point (including the edge of the rubber roof). Pool noodles custom cut for every sharp corner. Added extra tie-downs to secure any loose cover material. It's a lot of work. If you don't take the time to do the prep work (especially in very windy areas) the cover will self-destruct, not to mention damage the exterior of your TT.