Most of the fabric stiffeners we find in the craft shops are water-based acrylic/glue proprietary formulations. I've used both Stiffy and Paverpol and there is no comparison. The Paverpol will dry rock hard when used with natural materials, like a cotton stockinette. One reason people are so excited about Paverpol is that it can be used for outdoor sculptures. Paverpol can be used on synthetics but you don’t get the same stiffening qualities. On synthetics I found it similar to the results achieved with Stiffy.
There are other stiffeners available. The millinery industry has a felt stiffening agent available , although I believe that most of these are shellac based. An acrylic based stiffener is offered by Golden Artists Colors, it is called GAC 400. It is primarily used for stiffening canvas but as long as you aren't making wearable art it should work fine for natural textiles.
Sherry has used Modge Podge and Petal Porcelain as stiffeners and she said they work fine. I have heard that Modge Podge tends to stay a bit tacky in humid conditions although that is primarily in regard to paper products.
Both Modge Podge and Petal Porcelain are made by Plaid. The Plaid web site states: Petal Porcelain® is a water-based medium that is milky but dries clear and gives body and stiffness to fabric. This is the products that will turn your silk flowers and ribbons into decorations that look like porcelain. It will also adhere your silk flowers to the item you are decorating.
Another thing some folks do is water down either white glue or acrylic gel medium. It just depends on what effect you are looking for. You can use multiple coats of Stiffy also.
Hope this helps!