Glass tends to add stiffness (try bending one of my ffhlg wings ).

It is a matter of measure as well, glass does not add anything like the "strength" of carbon.

But what must be remembered is that neither will support a substrate beyond that element's failure.

A cf spar will rip through a foam core if stressed too far. Glass will crush foam. A cf spar will destroy a built up balsa wing (I know that one ).