the 20 gauge is much easier to tip (fold) the edge over the tailgate inner frame.
So it would definitely come out better for a bloke with not much experience with panels in his own shed,
giving it a go by himself.
first time i tipped a doorskin ,it looked horrible .
hammer marks everywhere and my boss screamed at me
it would have been even worse if it was 18 gauge.
it is also quicker to get the compound curve on the skin with the english wheel,easier to shrink and stretch the edges
thats just my thinking anyway after working for a couple hotrod/restoration shops ..
not trying to talk crap just giving my assumption of the thinner panel
regards wayne