Let's move off the "pop quiz" (which I've been taking the questions and answers from) and pick through the guide for some info to ask a questions about.
Next to your rifle and your ammo, what is the most important part of your shooting equipment?
Once you get that answer, which of the 3 possible answers is the best?
sling
Yup, it's the sling. Now, of the 3 types of "GI" slings which does the guide say works out the best. The three types are:
1907 leather
cotton web
nylon web
The guide is detailed/specific with the reasons for the one considered to be the best choice of the three.
nylon web
That's it.
The guide recommends the nylon over the cotton web sling due to it's durability (wear and tear) and resistance to mold/mildew in wet conditions.
My experience (what little there is) agrees with the guide. I have not had an issue with the nylon sling slipping down on my arm (I use the hasty sling all the time).