I have both and I prefer my silk pillowcase. My satin one was around $4 at Bed Bath and Beyond and is just OK.

My silk pillowcase I found on Amazon for $19.99 and I really like it. It does a nice job of not taking moisture out of my hair (like my cotton pillowcases do). I can also get 2nd hair if I put my hair in a pineapple and sleep on the silk pillowcase, because it doesn't rough up my hair, something that I can't really achieve as well with my satin case.

If price isn't a barrier, go for silk. If it is, satin is still better at reducing frizz than a cotton pillow case.
Curl Style: 3b/3c
Texture: Fine
Porosity: Normal

Co-wash: Suave Naturals Coconut
RO: GVP CB
Styling: As I Am Leave In, Jessicurl Rockin' Ringlets & Confident Coils, DevaCurl Set Me Up, Curl Keeper
Gel: Ecostyler Olive Oil, BioSilk RHG