As much as I know Paul has good stuff, don't get a fiberglass bumper . Get the real thing from Japan from Shawn Gilson at http://www.challengerdeep.com and you will never have to worry about it cracking in the cold (like my replica version 5 spoiler ).
I have to wonder about the quality of fiberglass workmanship in some of the replica products. Fiberglass is not the problem. I used to work building race and ski boats for my uncle many years ago, and I have seen boat that we built still out on the water running. Yes the temperatures do get very cold during the winter, and the boats are stored outside generally. I have seen a few that the gelcoat began spider webbing after a few years, but it just baffles me that spoilers and bumpers are cracking.
It isn't hard to wet lay fiberglass and make it strong and resilient. I have seen a few replica pieces that I was highly impressed with the thickness of the fiberglass on. I think that it is ok to get a fiberglass replica piece, as long as you can check it out for quality first.