Just thought I'd reply to this since I just ordered a custom set and went through the process of figuring out why some photos I see of the pad has a pronounces S curve while others don't. Basically the stiff core is the one that has that curve, the regular core is the one pictured in the customizer and more straight.
If I'm not mistaken this is the only pad at this time with some curve stock, and why I wanted to try Brian's again. As I've gotten older my hips lost some rotation and I feel like that S curve really helps seal it up. My custom order is in for an early March slot so won't be long before I'll be able to compare for myself. Currently in a True PX3 and before that was in the stiffest core Warrior G6 so will be a nice change of pace I think.
Picture attached is stiff core on left and regular core on right to show what I mean. Shout out to Jay at TGC, I've bought like 4 or 5 custom sets through him now and he always takes the time to go over the specs for me and help me decide.