I sit in a C and it's still a smidgen too short for my lower leg length. So, all I really know is that Bs are too small, and I've spent so little time sitting in a B that I cannot even tell you how. ;)

The difference I notice is the height, and perhaps the size of the seat back, but there may be other differences as well.

I did learn the secret for figuring out an Aeron's size: Under the edge of the back of the seat, stamped into the plastic, are one or more raised dots, like Braille. The one with three dots is a C. The one with two dots is too small for me, and I've never encountered a single dot.