A lot of the fiberglass fenders don't always fit that good. With a steel fender you can slice and dice and weld to fit. The procedure of doing that on fiberglass is a little trickier. An experienced body man working with fiberglass can make it happen.My choice is to always go with steel fenders. My buddy drives his '52 Chevy pick up year round in the snow and salt up here in Boston. He put fiberglass fenders on, so he'd have no worries with rot. The fenders don't look that great, but it's a daily work truck. From 20 feet away the fiberglass fenders look OK.
