Here are my thoughts on the tonneau covers I've owned over the years...
Lund Fiberglass on an 01 Silverado. Positives - Other than the basic idea of protecting contents from the elements, I can't think of any. Negatives - Heavy, unable to stand or store items on, huge PIA to take off. DON"T EVEN CONSIDER is all I can say about these old school fiberglass covers.
Line-X (Diamondback SE) on a 09 Silverado. Positives - Tough, can stand and jump on top of it (I'm 6'3" 225), the front and back fold was convenient at times, good seal. Negatives - Not flush with bed, not the prettiest, makes truck look like an industrial vehicle (not what I was going for), not terribly difficult to remove, but takes more time and effort than all but the fiberglass cover.
Extang Solid Fold 2.0 - Positives - Not as easy as Armor Flex to open (but still easy IMHO), didn't seem to scratch, lightweight, easy to remove, good seal. Negatives - Not flush with truck bed, moisture can freeze between cover and tailgate making tailgate to tough to open.
Undercover Armor Flex - Positives - Easy, shouldn't scratch (I've only had this for a month), lightweight, easy to remove, good seal, flush with truck bed. Negatives - Seal is a little crinkly (doesn't lay perfectly straight, flat).
In my experience here in Michigan, freezing moisture can/will prevent some function of all tonneau covers listed above.