The Forum Republic is the stiffest of the Forum line. The baseplate is made up of nylon with 30% fiberglass. If you want a bit more response out of your binding then grab the Republic.
This year, it has been upgraded with Good Vibes tech. The hinge disk allows the binding to flex 71% more than every other 4 hole disk binding. This gives you more board feel and control. You will love the Good Vibes tech for your 4 hole disk board.
The Ankle strap and toe cap are great. Really comfortable, and very durable. The toe cap is a eva wrapped PU spine. It has yet to fall apart on me.
If you want a bit stiffer of a binding, but don't want to give up board feel. Go with the republic.