I'm considering getting a pair of Andys Underwear to go under a Oceanic Aerdura drysuit. The primary diving temperature would be in the 40-50F range. I'm generally not a cold person, I can dive in the range in a 2-piece 7mm and be comfortable. My primary debate is between the US100 and the US75 (although I would consider something else as well). I've seen samples of the material, and the US75 seems to be considerably thinner and according to the website there is only a 4F degree difference in the "acceptable" range.