I prefer the double tire cover as it covers my RotoChocks too. This hides them from thieves and keeps the sun off them as well. I had the white covers but the soon looked ugly from dirt and grass stains. When they wore out (cracked and brittle from the sun) my second set is a black set and they are great so far. I haven't had the black set long enough to comment on the sun exposure yet.
One extra note......if you go with a double tire cover, the slide out may have an adjustment bolt that sticks down low enough that it can hang on the tire cover in the middle. I had that problem. I solved it with a bungie cord around the tire cover middle on the slide side.