I had a Leer shell on my GMC and loved it. Very secure and looked great.
My new Silverado has a Retrax Pro XR coming in today that I'll install next week after my liner is sprayed.
I went with the Retrax Pro XR since it has t-slot tracks for roof style rails (Yakima, Thule, etc) for tents, baskets, and all types of other mounts. It's also heavy duty aluminum with ball bearings, and the cover can lock in any position as to let tall items stick up by the tailgate but secure everything else.
I've had, on the same truck, a Roll N Lock retractable cover, a Gaylords hard, color matched tonneau, and a Leer 100 series shell. All were great, and in general I liked the shell the best. However, I also had another truck and a flatbed trailer.
I went with the Retrax because of the overall versatility of it. My bed is just under 6ft long, so the cartridge real estate isn't a big deal as I already have a trailer. It's not the cheapest one to buy (I got a holiday deal at $1,960 out the door) but I really wanted a low profile and strong cover that looked clean. Additionally, I think the t-slot tracks will be great on longer trips since we have a family of 5. If I need to ass a cargo basket over the bed I easily can, and if I decided to get a RTT again I can easily mount it, and the fact that it's below the cab roofline means it will not mess with MPG or drivability, especially in winds.
I hope this helps!