Hi Barkas
We have had covers for all of our caravans going back about 15 years. The very first one was like an opaque giant plastic bag and was known in our house unsurprisingly as The Condom.
Our driveway is overhung by a Lime tree growing in the public highway verge on the other side of the fence. The drippings from the tree during the summer months make it a nighmare to keep the caravan clean without a cover.
More recent covers have been supplied by Specialised and Pro-tec. Both suppliers' products have been very good and both firms have, in our experience, been a pleasure to deal with.
I have to say that the green material that they both use seems to me as a layman to be very similar, even though they will each lay claim, no doubt correctly, to a unique product. It is our experience with both that after about three or four years the material becomes significantly more brittle and cracks start appearing (and even small tears) where regular folds are made. As we have never needed one to last longer, we usually find a strong desire to acquire a new van at around 4 to 5 years, I can't say how well they last beyond that timescale.
Hope that helps,
Mike A