I noticed my cousins vette 81 has a fberglass rear spring while my 82 has the steel which of the two are the options on vettes?

According to the Corvette Black Book, the fiberglass spring became standard in '81 with cars equipped with an automatic and standard suspension.

The composite spring is much, much lighter and is supposed to last longer. Unless the car has been really abused or an ungodly amount of miles on them, either should last the life of the car. If you've done suspension modifications it might be a reason to install new springs matched to the new suspension.

The composite spring is variable rate, which means the car will ride better, and corner better.  It is a bit more expensive, but you only replace it once.

Larry