When deciding on a new (or first) garage door, there is so much to consider that the material it's made out of hardly seems important. It's much less exciting than choosing colors, styles, and garage door openers. But it can make a huge difference, depending on the weather in your area and what you use your door for.
Steel has immense longevity, has the best security, and it's very easily insulated. Also, keep in mind that a steel garage door will not look like steel- they are usually covered with exterior coatings that can make them look like wood. The downsides are that humid climates tend to rust steel doors, especially if you live near a warm ocean with a hot sea breeze. They also dent more easily than fiberglass, and this garage door repair can be expensive if it's important to you to maintain appearances.
Fiberglass garage doors are actually something of hybrids, built over aluminum frames. They're very lights, which puts less strain on the motor of your garage door opener. However, thye tend to be flimsy and difficult to insulate, and fade pretty severely over time. A safety concern to keep in mind is that cold climates can freeze fiberglass, and when that happens they can shatter pretty easily.
Pricing-wise, however, it's no question. Fiberglass doors are much less expensive than steel doors and cost less than half as much.