I use my pick up only for camping/travelling. If I have go somewhere far, sometime 2000 km, I drive for 12-14h and stop at a hotel, not in mood to setup a camp at night after all this driving and start quickly in the morning with the driving again.
So I have to have a secure place for the stuff I carry. I have an RTT so the shell must be strong enough to suport it.
I chose an aluminium one, made a by a south african company, Bushtech. If it is good enough for the roads and conditions in Africa, is good enough for me. Very strong frame, I saw a video where they put a Landcruiser 79 on top of it.. :)
Access is easy as the flaps open from all 3 sides on the full lenght, good dust and rain isolation, they can come with a special tailgate dust kit ( that will be the vulnerable spot after you install it )
On top you can go for roof bars like me or have a roof rack, comes with rails that can support almost any type of bars or legs for roof racks.
If you go for a canopy , and specially if you plan to put anything on top, make sure it can take the weight. Most standard fiberglass ones don't cut it. A not so easy to install steel frame is required for many of them to be able to use the roof. An aluminium one is a bit more expensive than a good quality fiberglass, but not my much. In Europe a good fiberglass canopy is arround 2800 euro, an aluminium one is aprox 3100, so not a huge difference.
The main limitation is what you can put in the bed of the truck if you use it for other things than camping, otherewise your stuff is more secure, dry and dust free.