The only reason I don't do a cap is I use my truck bed for other uses. Loading a dirt bike, helping peeps move, haul alfalfa and it's nice to have an empty bed to just let her get wild sometimes.
(But caps are bad ass because you can keep your belongings safe and secure.)
5' 6" bed, so I went with the 52" rack. The only issue I have is that when you go to tighten down the rack to the bed you will deform the top of the bed rails. (They will concave a little bit)