In broad terms nylon:-
is heavier; has more stretch in use and can be permanently stretched; absorbs of water; is cheaper; has colours that don't last as long.
Yeah, but no but yeah but ... OK, imagine you're a kiteflyer who's not an expert, one who really just wants to fling the thing about the sky for fun at the weekend, and maybe - just maybe - to try one or two of the clever things done by people with [a] £290 kites and [b] skill.
In those circumstances would you buy a pretty, £84 kite (with a nylon sail) that aspires to be rather similar to a £290 kite made of Icarex/polyester? Or would you tell yourself that prettiness isn't all that important, and shell out £98 for a rather plain (but Icarex-sailed) kite from the same manufacturer?
I'm currently very attracted by prettiness. But I just can't persuade myself to commit, if flying nylon is going to be like flying a cotton shopping bag with a readymeal in it.