This question comes up constantly in natural hair spaces and the answer is more practical than most people expect. Both satin and silk protect your hair from the friction and moisture absorption of cotton. The primary difference between them is what they're made of, how much they cost, and how you care for them. For the majority of people, neither is meaningfully better for hair protection than the other.
What Silk Is
Silk is a natural protein fiber produced by silkworms. It has a naturally smooth surface and contains some amino acids, fibroin and sericin, that some researchers believe may have minor conditioning benefits to hair. Genuine silk bonnets and pillowcases range from about $30 to well over $100 depending on quality. Silk requires delicate washing and doesn't hold up as well to machine washing or tumble drying.
What Satin Is
Satin is not a material: it's a weave pattern. Satin-weave fabric can be made from silk, polyester, nylon, or cotton. When hair care products market "satin" bonnets, they almost always mean polyester fabric woven in a satin pattern. This costs $5 to $15. It's machine washable, durable, and widely available. Its smooth surface creates the same low-friction environment for hair as silk.
Does the Difference in Material Actually Matter
For the primary purpose, reducing overnight friction and limiting moisture absorption from the hair, no. A polyester satin bonnet does this as well as a silk one. The amino acid content in silk is present in such trace amounts in a pillowcase or bonnet that any conditioning effect is essentially cosmetic marketing rather than a meaningful benefit.
Where silk genuinely has an edge is for people with sensitive skin or scalp allergies to synthetic materials. Natural silk is hypoallergenic and breathes better than polyester, which matters if you tend to overheat at night or have scalp sensitivity.
The Practical Recommendation
If budget is a consideration: buy a polyester satin bonnet. Two or three of them so you always have a clean one available. The protection is real and the cost is minimal.
If you want to invest in a pillow upgrade: a satin or silk pillowcase is a good complement to a bonnet for nights when the bonnet comes off. Even with a bonnet, the back of the bonnet rubs against the pillow, so a smooth surface there helps.
The most important thing is simply using something. A $5 polyester satin bonnet worn every night does far more for your length retention than a $100 silk one that sits on the nightstand most mornings.