wouldn't the benefit of the hex help secure the extension?
The hex-head will help prevent rotation around the extension's long axis, yes. The hex- and round-head are both equally secure IN the clamp. The only question is whether the mini rotates along its long axis. FWIW, I've never had a problem with the mini rotating on me. Just need to tighten the set screw securely. I think the infinitely variable rotation of the mini is well worth it.
If you go back and look at the pictures I posted, the mini-arm will have the same benefits and side-effects as my RA/stud combination. You just have to screw it on rather tight to ensure no rotation (which is quite possible), where as the hex stud you can snap it in, and screw it down and *know* it's not going to rotate. That said, you are more limited in your angles of use.
For the RA/stud combo you see, I've screwed it (forcefully and intentionally) over and over in the same spot so now if I'm careful when I get to where the screw touches the stud, it will align and correct itself since I've created a divot where the screw pressure is. The only
worry comes if your going to have unbalanced weight on the end of it which will torque toward one direction and give it reason to rotate, and even then I've gotten off center with mine when weighted before with positive results. Just tighten it up good, it won't move if your careful about it.