I don't know which cover is most durable BUT I ordered a Fan-Tastic Vent U1500WH Ultra Breeze Vent Cover to cover the Fan-Tastic vent on our scamp
. When I placed it over the vent, I thought it was butt-ugly. We have a Brisk 2 air conditioner on top and the cover really contrasted with it. Also, it has the imprint of a vent cover molded into the front of it, which I didn't like. And, I would have had to drill holes in the roof to attach it with rivets. I sent it back.
I then bought a Maxxair Fanmate, model 855, for $20 less than what I paid for the Fan-Tastic cover, and I love it. It appears very durable, requires no drilling into the roof, attaches with brackets mounted on the sides of the Fan-Tastic fan, can be easily attached or reattached with four pins, can be opened to either side, and it matches the Brisk 2 AC. It took about 1/2 hour to install this morning.
Go with the Maxxair!