In what kind if pieces are you referring to? Both have great advantages.
Fiberglass is light but very durable if laid right, and used for things like helmets to keep costs down, not to mention retain screen accuracy.
Aluminum is used on many small parts of the armor and people prefer it because it has a "real" look and feel.
I know there's aluminum armor out there, and even fiberglass armor. I personally wouldn't do either as its not accurate and the vacuum formed armor is very rugged.