Some of the responses seem to imply that a fiberglass boat will somehow be damaged if it hits a rock too hard. Nothing could be further from the truth. Ron Laviguere of Lavro Boats has a video where he drops a 50 pound rock onto the bottom of his boat from a height of about 20 feet and it just bounces off. I've been in the shallowest and swiftest water in a Lavro and hit rocks hard enough to drop teeth..just an occasional scratch like you'd see on a metal boat. There are differences, but to say that fiberglass is somehow less sturdy is inaccurate.
I've been in about every model of fiberglass and metal boats and about the only plusses and minuses I can say are this:
1) Metal boats sometimes track better than fiberglass when pulling plugs.
2) Fiberglass boats slide over rocks easier and are much better in "skinny" water.
Since I'm admittedly fiberglass biased, I'll add one more: Pulling a boat upriver for a mile (like we had to do on the Hump once) will definitely make you prefer fiberglass to hulking iron!