We have a cap and it's been the best thing for travelling. Suitcases, coolers, gear, golf bags, folding chairs etc. It has a hanging bar for clothes on hangers.
The only thing that goes in the back seat is small coolers with road food and water.
I still use the bed to haul bags of mulch, dirt etc., but obviously with a cap hauling bulk is now going to happen.
We have the plastic bedrails AND the installer also put in some foam type gaskets to sealout rain, dirt and dust.
It locks, but if someone wants in bad enough they can break windows. We had a tonneau on a previous truck and you could stand on that. We like it...but it had drawbacks..space being the biggest issue.
It depends on how you want to use your truck.
cts