One consideration I have when evaluating the desirability of a slipcase is whether the covering material of the book is durable enough to withstand multiple insertions and removals from the slipcase. You often see editions that haven't accounted for friction - books and dustjackets get frayed or crimped, or the surface of boards get scuffed, simply because the book wasn't designed to be put in a slipcase. Or if the boards of the book are bound in a few different materials, the "seam" where the materials meet gets damaged because it catches on the slipcase edge.
One advantage of traycases is that you can usually remove a book more easily without worrying too much about wear and tear on the edges of the book.
HBJ