The "B" mount is more rigid and is more adjustable but a is fixed length (11" clamp end to bottom of camera). Care must be taken adjusting it as metal teeth grip the rubber ball at the clamp end
Note: The largest tube that I tested the mounts on was 2.88" in diameter.
This is larger than the largest crossbar that I know of which is 66mm or ~2.6"
(except for TRX crossbar on which the mount won't work). The smallest tube
that we have clamped to was a 3/4" washout strut.
7075 tubing will flex so that if you tighten the mount too much, you deform the tubing which means that you don't want to use too heavy a camera. The clamp/camera could sort of squeeze off the tubing OR cause structural problems if you overtighten the clamp --- you can deform 7075 T6 66mm ~.035" with hard finger squeeze. For the loads that it is designed to take the tubing is great but clamp abuse is not advised.
The preceeding is why we came out with the "Heavy duty" mount for those using relatively heavy cameras. Before we made the heavy duty ("C" mount) we had "gotten away" (nasty, dangerous term) with using the B mount w/ a light camcorder on keel and some have done it on the crossbar but it really is not a good idea.
The "C" heavier duty mount distributes the load over a larger area and has more than twice the grip. It has been used for about a year on a variety of gliders and all those who have given us feedback, like it a lot.