My understanding is that the OD cotton sling is most grippy (a highly technical term) among the cloth slings, so it remains in place on your arm more reliably. Anything else (e.g., black synthetic) is considered slick (again, a highly tech term). I do like the slings from Tech Sights or a vendor called At The Front. Of course, many other options available out there.
Having said that, I’m sure others have likewise useful observations or experiences.
Having said that, I’m sure others have likewise useful observations or experiences.