The Carefee Cover King III or the Dometic Elite..
Would that be the Carfree Slide Out Kover III? Coverking is a brand of automobile and seat covers and I don't think they do slide toppers.
It's pretty much a toss-up, I think. Carefree uses a good fabric, especially if you get the [more expensive] woven acrylic instead of the vinyl, but they have had problems in the past with the stitches rotting away after several years. Hopefully they have resolved that problem by now, but it may be more years before that is proven. The Dometic works decently but [in my opinion] has a shorter useful life than the Carefree. Much depends on usage and the weather they are exposed to, though, so hard to generalize.