What is the difference of using a steel vs a fiberglass door for the exterior of my house
one is made of steel and the other is made of fiberglass.
Seriously, you might look at the thermal rating of each but that would depend on the specific door construction. Steel will dent if it gets hit with somehting and it will rust if there are scratches in the finish to bare metal. They can put a texure in fiberglass to make it look like wood.
But the primary difference between the two is in the material itself.