We have a Leer cap (called a "canopy" out here) for our Tundra and really like it.
I think the real question is how much of what type of gear do you need to take with you?
Part of our reason for wanting a cap that is flush with the roof of the truck is that we often take the canoe with us when we go camping, so it facilitates putting up the roof-rack. With the canoe, we also need to carry life jackets, paddles, and oftentimes portaging wheels and a plastic barrel for lunch, jackets, &c.
For longish boondocking trips, add in the generator, gas can, jerry can for water, folding chairs, camp table, Dutch oven, and so on, and pretty soon the larger cargo space starts to seem like a good idea.
If the truck is full of passengers, the capped bed also provides a reasonable place for our mid-sized dog to travel in.