With our first trailer, we weren't sure whether or not covering the trailer was the way to go for us, so we got the cheapest one we could find which was an Expedition brand made of a non-woven fabric. It did the job, and kept the trailer looking brand new and clean between uses, but the cover was cheaply made, and it pretty much disintegrated after a couple of years.
Since then, we have used CalMark brand covers, semi-custom made in the USA to your trailers measurements from Sunbrella woven fabric (also made in the USA). These covers are more expensive, but hold up really well.
Other than the quality of the materials used to manufacture the cover, proper fit is the most thing. If it is too small, it will strain the fabric when you put it on, and it will be difficult to get off. If it is too big, it will be difficult to secure it properly, and the wind may cause it to rub the trailer finish.
The point is, depending on the actual measurements of your trailer, one manufacturer's "standard" cover may fit better than another manufacturer's standard cover ....
Bill & Kate with Zeke (parti-poodle) & Bailey (std poodle)
Stone Harbor, New Jersey
2014 Forest River Wildcat 272RLX Fifth Wheel
2018 F-250 4x4 Crew Cab 6.2L Gasser