Picking up on the point about why one airfoil is so "better because...", I have often wondered how much "fashion" has to do with this.

Because a particular model is used in winning world champs, and because that model uses section "X", then the world (or a goodly part of it...) assumes that "X" must be a dang-fine-numberone type airfoil. So it gets used on the latest scratch-builds, then kits and arfs, and all of the factors that have been listed here in previous posts are ignored.

For someone like me, the side of a gumboot might be just about as good .

Oh, BTW I just found out that the airfoil "X" used in the WC winning model had an error in the parameters for the foam cutter, and consequently the wing looks nothing like it should by about 2mm on a 250mm chord.