for me it’s pretty clear, to have the full experience, smell and surrounding/looks are also very important. so I will always go for natural fibers. I had some other robes, but i have thrown them away. Now my collection is mainly cotton, jute and hemp.
Cotton, is the soft one, leaves almost no marks, so can be very handy if you have to be carefull. but it slips a lot and is weak.
Jute is my favorite rope kind. love the smell/feel etc. but still it is not the strongest naturalrope.
Hemp is my main suspension rope kind. it’s strong, feels good, bit rougher then the other two, but great to bind with.
next to those three I also have some very rough natural rope made of a special grass kind. not comfortable, really strong and will give burns. because sometimes it’s also nice to punish with bondage;)