I sometimes use hemp, and/or jute, IF I intend to make a braided sling. Sometimes I use nylon or poly-whatewer cord, if what I got available is of suitable size I tie it on to a leather, safety belt or other kind of pouch, if it is to thin I simply braid it.
I do not consider one way or material better than the others, I simply us available or interesting materials in the best way .
I have made usable slings from sisal and tough grass, too, and think they can be made from almost anything available
timann