read this somewhere........s100 is targeted toward bike owners and p21s is for cars. The difference is the clarity although it is not much. There are two types of carnauba. there is the whiter looking carnauba taken from young plants and the yellow is taken from the older , more mature plants. the s100 has a higher concentration of yellow carnauba, which also has more durability and can withstand higher temps than the whiter carnauba. The p21s is mostly a lighter carnauba which is also more clear making it ideal for car finishes. This is my two cents....if you hold them side by side they are just slightly a shade off...not much diff. I would go with s100 for greater durability since you would be paying less without much of a difference in any other aspect.