If the goal is to improve insulation, a fiberglass door is definitely a good choice. We replaced our old wooden door with fiberglass doors, which we bought at
https://www.elegantentrydoors.ca/entry-fiberglass-doors/ , and now we spend considerably less on heating. The door looks stylish and feels secure, and we really like the look of the door. Ordering was easy and they helped us with the installation. In addition, they advised us to choose a coating that increases protection from moisture and cold, so now even in severe frosts the house stays warm. I recommend checking them out!