I always try to err on the side of caution, so I bought the large hook, even though the medium was rated to hold what I intended to use it for. 
The hook worked well for a while, and I was quite happy with it, because it provided an efficient solution.  That is, until this weekend, when I found everything on the floor, including the hook with a piece of my wall attached. I would have been better off just drilling a hole in the wall, it would be easier to repair.