If your goal is to have the absolute lightest ov either... get a 70 Duster and make it into either 71. They got heavier year by year... and between even 70 and 71 there were many differences that all add up. If i was going to build a 71 Demon i'd buy a 70 Duster and clone it. Easier to find, cheaper, and lighter. As far as just the two choices you listed though... the Demon does seem to be on average (posted weights) about 15-20lbs lighter... which makes sense to me looking at either car.