I've always thought they were the same. I recently read a post on some forum where the poster claimed that the S100 had some kind of elasticity added to give it some flexing properties, but the p21s didn't.
I tend to think they are the same product, but haven't used both side by side to truly give my opinion.
I do know that autogeek.net had/has the 5 Liter p21s for 49.99, which seems like a pretty sweet deal - if you like the product.
Cory