I have a Pace Edwards Full Metal Jackrabbit because I do have a 5er and can pull the cover back over the hitch when not towing (which is most of the time) to hide whatever I may have in the bed. The last time I had a cap (many years ago) it was gasketed all around so it protected the truck rails from any rubbing.
From my experience/observation, a cap will keep things dryer than a retractable tonneau cover as there is more of an overlap of the bed all around. My Pace Edwards does a pretty good job, but it does allow some water to enter where it pulls back against the tailgate. Just have not found a way to completely seal that area at the corners of the tailgate.