There are also differences among them in the quality of light, some more specular, some more diffuse, and in the colors of the gold and silver used. Some also have mixed stripes, silver+white, silver+gold, and many other variations. The translucent diffusers also have different degrees of diffusion and transmission. Some are more triangular and have a nice handle built in. Build quality and materials quality varies. Read the catalog descriptions, look at the structure, try to see them in person if you can.
The Calumets are good, as are the Wescotts and Photoflexes. I have a generic from I want to say Promaster (or it could be an Interfit) and it seems ok too, but for the usually minor difference in price, I'd go up a grade to the Wescott or Calumet products.