The fabric. Polyester corsets are going to get warm really quick, whereas cotton is more breathable. Durability really depends on the quality of the weave, but anything with the name polyester/rayon blend is garbage. The most important thing to consider when shopping for corsets is not so much fabric as the boning, if you are looking for cheap halloween costume quality then it doesn't matter. But if you want a TRUE corset that is going to reduce your waistline, if it doesn't have steel boning, you are wasting your money.

The names.