Well that all depends on the system you use. With traditional wrap systems we have always believed the following:
Silk is a natural fibre and therefore it soaks up the resin and becomes one with it.
Fibreglass is a man made fibre and stays intact while the resin 'swirls' in and around it. Rather like metal mesh inside concrete blocks. It provides a little more flexibility.
Silk has always been considered the weaker of the two.
HOWEVER ... new technology is always right around the corner and people have to learn to abandon some of the 'old' beliefs in the place of new ones.
With the Fabric# system there is no discernable difference in strength between silk and fibreglass, they are equal. It then becomes an aesthetic choice of which look you prefer. Fibreglass is very clear and crisp and silk provides a softer more gentle look.