Just a warning; if you DO use fiberglass pins, you MUST tie the rudders onto the boat as a backup. The Chesapeake Bay still holds a set of EPOs and castings and a hot stick after both pins broke on my H16 at speed...
For the same material at the same WEIGHT, a hollow rod will be stiffer than a solid one. It will be a bigger diameter in order to match the solid rod weight, and THAT'S where the extra stiffness comes from.
20's are famous for bending SS pins. I always just rotated the pins and used the rudders to bend 'em back so I could pull 'em out. I figured if the transom was strong enough to bend 'em, it was strong enough to bend 'em twice.

I threw the bent ones away; learned the hard way that they don't bend three times without breaking...