(All this information comes from things I have researched or been told by other "experts")
Weave: This is the most important factor when choosing among satin sheets. NEVER buy "knit" materials. They might be made of a shiny thread, but the surface will be rough. Almost 100% of the "satin sheet sets" you see in department stores are made of knit acetate. When you get them home and put them on the bed, you'll see the knit surface is not sexy at all. Knit fabrics may be OK for satin bedspreads or satin comforters where looks are what matters, but never for a sleeping surface.
What you are looking for is "woven" fabric. It's worth the (significantly) extra money. But this is the only way to get smooth, high thread counts. There are other manufacturing methods as well, but almost all leave you with a less-smooth surface than a woven material.
Fabric: (the following information comes from Fabriclink.com)
There really is no "best" kind of satin for sheets. All the following materials can be used to make high quality, smooth satin sheets. Different companies make all sorts of claims about which is best. Read the characteristics below to decide for yourself.