Never actually heard a sub in fiberglass, but I think it really depends on what your goal is. Fiberglass is much more custom, if you need to utilize space most efficiently it is the best choice I would say. As far as sound goes, I'm not the one to answer...yet. Give me about 2 or 3 weeks and I'll tell you my opinion though, I just ordered my fiberglass stuff and should hopefully be done by then.
If the box will have large flat surfaces, then those must be made of MDF. The fiberglass will flex and resonate in those areas and color the sound. Usually the speaker baffle is made of MDF and any flat surfaces that can be made of MDF are on a fiberglass box. If the box is made right and maximally stiff, it won't matter which material the enclosure is made of; they will sound the same assuming the same sub, aiming, enclosed volume and car.
Either or brother man. Just depends on the situation and space and shape of the space availiable. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif