B springs are progressive rate springs and slighlty stiffer than stock springs.
C springs have the same progressive rate rear springs as the Bs. The front springs are a linear rate spring 600lb/in and are much more stiff than the Bs.
I went with B springs because I wanted a lowered stance with a as close to stock ride as I could get. They are still plenty rigid. At high speeds the car doesn't feel unsafe anymore. And, with the Strange 10way adjustables, I can dial my ride for comfort or racing.