I am thinking of buying the GM tonneau cover for my 2010 silverado. The price is about $50 dollars more than everywhere else. It has integrated support bars which I like. Is it worth it? Does it sit on top of the rails or inside them? Does it use velcro or tongue and groove? Does anyone have one and recommend it. I like the folding ones too but they only fold 2/3 of the way and with a crew cab short bed the bed is small our thoughts?