I agree, you can "push off" a stiffer fin (100% fiberglass) better in weak waves, but main difference I've noticed is in "cant". Super canted fins need to the board to be put heavy on the rail in order not to slow it down. That's why I generally prefer more vertical fins on slower waves - they glide better. Some people say it's all too negligible to matter, but when you're grovelling, everything adds up
Anybody who says fins are fins and don't matter is in fact the real kook, sorry for the language.