Brian,
I have a fiberglass tonneau cover. I had been planning to get a cap while the truck was on order, but I changed my mind at the last minute. Sometimes I wish I had a cap instead, and sometimes I wish I had nothing on it at all. Here's my thoughts after almost 2 years. But mostly, the cover works very well for me.
Pros: Locking it secures the bed since it hangs over the tailgate; Nobody can see what's under there (tools, shopping bags, etc... ; A little better visibility while driving; Everything stays completely dry under there.
Cons: Obviously limited as to what I can haul (height-wise); Heavy - would need at least 3 people if I want to remove it (although, this is also a pro in the snow); it's beating up my bedrails a little bit.
Sometimes I think about going to a cap, but the pros of the cover kind of outweigh the cons for me right now. A roll-up would eliminate my cons, but would also take away a couple of the pros. I use my bed pretty-much as a trunk - luggage, tools, etc... I don't haul gravel ar anything with it. I do haul firewood on camping trips, but that's about it for raw materials. A carbon fiber cover would be the ticket for me - something solid, yet lightweight and easy to remove.
Dave