For what it's worth, I just got the Medium length Flexible wraps and they are beyond sufficiently supportive for the average lifter's heavy pressing. They are far more rigid than, say, the basic Rogue or Harbinger sort of wraps.
I was hoping for a more nuanced answer. I'm trying to figure out, if the stiff ones are the real shit and the flexible ones are merely an accessoire for vain gym rats. Similar to the cheap belts you get if you by half a ton of protein powder vs 13mm INZER belts.
My wrists do hurt at times and become a bit unstable under heavy presses. I figured they could use some support.
I've never tried the stiff ones. But the flexible ones can be made as soft or stiff as you want them. The tighter you wrap them, the stiffer they get.
I ended up bying the Inzer Wrist Wrap Z, which should be similar to the flexible SBDs, but cheaper. I've used them once and they are exactly what I needed. The thumb loop being on the same side on both wraps is stupid, but I don't use them, so I don't care.