Some people like nylon because they say it won't stretch over time like the cotton sling and it is tougher and lasts longer.
Some people like the cotton sling because they say it won't slip like the nylon sling will.
I use either and haven't had any issues.
Most folks will find they have to make a sling adjustment between positions. If you put the sling in the same spot on your upper arm every time as you change positions (standing, sitting, prone) your arm is not in the same place. We used to tell folks that once they found where the sling fitted best to use a black sharpie to mark the sling at that spot so they could more rapidly make adjustments between positions. Many folks have to make sight adjustments as well when they change positions (target shooters/high power shooters).
Be aware that as you practice more some things happen that may change how you need to adjust that sling. Your body begins to limber up, so your position changes some. Your eye gets to a different place behind the rear sight (because your body limbers up or because your body finds/settles into the most relaxed position to be in to improve your groups and your group placement on the target. You might want to wait to mark that sling till you know your won't be needing to change it do to finally finding "your spot" behind the rifle.