Silk is generally more sustainable than satin. Silk is biodegradable, renewable, and doesn’t use a lot of chemicals or water. That makes it a highly sustainable fiber compared to nylon or polyester, which aren’t biodegradable and create a lot of toxic waste during their production processes.
However, silk is an animal product, and thus it is not an option for vegans. There are also people who aren’t on board with the fact that silkworms are harmed during the making of silk. The cocoons are boiled or baked with the pupa inside, killing it to keep it from breaking the fibers of its own cocoon. If you take issue with this, silk may not be for you.