I think it depends on your preference. Some people prefer satin over silk or silk over satin. I think it is more so what your hair & skin likes. I personally prefer silk.
Silk hands down. I was wearing a satin scarf and changed to a silk scarf this week, because I have started changing my scarves when I wash my hair.
The change is dramatic. My hair is SUPER smooth and very soft, It feels so different than when I wear a satin scarf/cap. I did not have to moisturize yesterday (I washed on Sunday). I will never go back to satin.
I also have on a doo-rag cap that is very molds to the head better than the traditional ones it is very stretchy. I wear it over the silk scarf to help preven the scarf from sliding off. I keep taking it off to feel my hair because it feels SOOOOO GOOD!