Fiberglass is better due to the fact that it is lighter, stronger, and more flexible. Fiberglass can be used in many more applications than cast iron such as fishing poles and roofing materials. Fiberglass is also not a very conductive material.
Doing a bathroom update , the difference between cast iron or fiberglass is going to be weight and style. Fiberglass is exceptionally light, comes in many different styles. The soaker tubs with whirlpool jets are an example. Cast iron does not have the properties to be as flexible. Look at the old fashioned clawfoot tubs.