I have always used a bed mat, just to save whatever was under it from the screw heads that festoon the bottom of a Bigfoot. When I had the PE bedliner I had a plywood platform on top cut so it locked onto the bedliner, then the mat on top of that. With the LineX, I have a 3/4 plywood layer with the rails on it to keep the camper centered, then the mat on top. I thought maybe the PE bedliner which is kind of corrugated might be compressing a bit on the corners allowing the camper to lean and shift. But there was no difference with the LineX.
I've seen that Dualliner before. Seems like it would let dirt under it through the joints, then you might have paint wear? Maybe the ultimate answer is LineX with the dualliner on top.