They offer only first quality, premium brands at fair prices. These firms deserve your support. In conclusion, there is most definitely a difference in fabrics. You get what you pay for. Premium brands offer a vast quality advantage over cheaper alternatives for just a modest increase in cost, especially when you consider the effort, skill and love that will go into your use of the fabric.
Textile fabrication is a large subject.
If you would like to learn more about this fascinating subject, I highly recommend Harriet Hargrave's From Fiber to Fabric. It's a wonderful book, and you can find it in most quilt shops and catalogs. Jim Salinas, who has had 25 years of fabric chain store retailing experience, is now a sales representative for Moda Fabrics.