I don't think fiberglass can sound as good as mdf. You'd have to make sure it was thick enough so you didn't get any vibration from it. if you're looking to keep sq, stick with the mdf box. the only reason you'd want to go fiberglass is if you want something that will look really cool, or you want to build into a small corner to save space or something.
fiberglass is way harder and about 5 times more expensive, and fiberglass is usually sealed because its to hard to calculate volume to adust tuning. just do MDF
its not more expencive to go fiberglass rather then MDF. its just that shops will charge a lot more due to the extra time it takes to fiberglass up a box.