This is interesting, actually, because they are exactly the same silicone. The only difference is the color.
Generally speaking, the Supersoft and the outer layer of the O2 use the softest silicone. Most of the line (Echo, Silk, Vamp, Acute, etc.) are sort of a medium level. The really firm items tend to be toys made to target the prostate - the Protouch, Prostate Health. The Flex and the Handle toys are also made of the firmer stuff.
So if you're looking for the medium level, you're in luck - because there's a really big array of toys to choose from.