There is a market for fiberglass, but it is a small one. These enhancements work best (IMO) for natural nail wraps. They are extremely thin and it is very difficult to build an apex with this product because it has the consistency of water. I use fiberglass to repair a crack in a natural nail, then apply gel or l&p overtop. Fiberglass cannot be worn very long because it is so thin. Without an apex there is no strength.
L&P is harder, more durable and can be extended to whichever length the client is looking for. It is also extremely versatile as the market has many, many colors you can use. You can also learn how to make 3D flowers, do color fades and so much more.
I personally wouldn't recommend that you take fiberglass as a first course, as the vast majority of clients will be looking for l&p or gel.
Hope this helps