How much is it recessed? Fiberglass is less expensive than a tiled shower and can last a long time, but a properly installed tiled shower can be a lifetime install - a fiberglass one won't be. Depends somewhat on how much fill is needed, but there shouldn't be a problem filling it in. In fact, you could build a wooden platform to bring it level, so you could do the recessed, tiled, shower later without cracking out whatever is installed to fill it in now.
The only reason I'd use fiberglass verses a tiled shower is if cost was a big issue. The key here is a 'properly' installed shower, if done right, it should be problem free until you decide to remodel.