Lets just put it this way since I've built dozens of both. If you don't know what your doing in fiberglass don't waste your time. You have to know how to properly mix, size, frame, wrap, bondo, sand, paint etc... I used fiberglass for clients wanting to put subs in odd places. If your just starting out and don't care what a box looks like dont waste your time!! The thing to remember is the box is what makes the sub. If your box is flimsy or not built to spec (internal volume, port size/length etc) your sub is not going to be at its potential.
Building a rigid fiberglass enclosure is not easy to do. And measuring its internal volume is even harder. Building a box out of MDF and calculating its internal volume to match the specs is very easy to do. Just make sure its at least 3/4" MDF. Originally I started by fiberglassing the inside of my boxes but since I used to compete with small sealed enclosures it didn't make much of a difference. So from then on I just sealed everything with liquid nails as I built and then coated the inside with spray on bedliner.
