Woob is correct.
I have asked this question as well, I have answered this question as well, I have used these two products....here are my observations....
P21S - I bit harder consistancy in the jar....goes on smooth and silky.
S100 - I tinge bit softer in the jar, seems to go on more silky and buttery.
They both buff off the same.
I cannot tell a $10 difference (I think it is more now) between the two products once applied to the car. I have used it on both Silver Metallics, Olive Greens, Dark Carbon Blue and White colors.
I am sticking with S100....Cheaper, for all practicle purposes the same product. My gut tells me if there is a very slight difference in color and feel, it is the Flex agents within the product.
This is tremendous marketing! Free enterprise allows companies to hone the crafty skill of "How do we sell the same thing to different markets?" How many flavors of vanella are there? You get the idea.
Here is the one of the few Latin phrases I know....."Caveat Emptor"....." buyer beware".