I've got six exterior doors that were installed 10 ~ 12 years ago. Five are fiberglass, one is steel. Two of the fiberglass could stand to be repainted, but it is not pressing enough to motivate my lazy rear end to do it. The steel door is rusting at the bottom, and will need replaced. I guess I could install a pair of kick plates, but that will only prolong the inevitable. Rust never sleeps.
Perhaps this first hand experience will be of value when making your decision.