Fiberglass shower or tub liner vs. cultured marble custom?

We are looking into remodeling our small shower as well as the bathtub in another bathroom, which is peeling. One contractor suggested cultured marble walls and base with a new glass door, which would look really nice, but the price is several thousand. He also suggested replacing our tub and installing cultured marble walls around it--again, several thousand. Another contractor suggested a fiberglass shower and a tub liner, but I don't have any experience with either of these. He said you have to clean the walls of the shower or bath ever time you shower or the soap scum buildup will get to the point that it is impossible to clean well. This discourages me, as we have already had trouble cleaning the epoxy finish on the bathtub (scum seems to cling to it more, and of course now the finish is peeling off). The tile around the bath is in good condition, btw, and a neutral color, so if we do a fiberglass liner we might just line the tub and not the walls. But I am also concerned about the tub liner not fitting well/leaking, and about the tub or shower looking like plastic.

Has anyone had a fiberglass shower for any length of time, and are you happy with it? Since the bathtub is in the master bathroom, we're thinking of spending the big money to get that done very nicely, but the small shower is in a less formal, "extra" bathroom, so I don't feel the need to make it fancy. (On the other hand, does anyone have experience with cultured marble, and are they happy with that? Does it scratch easily, etc.?)

Suggestions? Much appreciated!

I should add that another option we haven't been quoted is new tile in the shower. Some places I have read that the cultured marble scratches and that buildup can stick to it more than to tiel, and I like the idea of something very durable. And some folks say that there is now stainless tile grout (our grout is now the problem--gets moldy very easily).

Or is the main problem in our family simply that the guys who use the shower don't wipe down the walls, and any system would function better if they would do that??

Have a look at this link...
Kerdi Mold Free Shower System Info
Probably the best out there today!

I have had a fiberglass shower for years. No problems. For a fiberglass (or any other plastic type) base you want a mortarbed under it so it doesn't flex. If you are getting mold, this tells me you need better ventilation. No surface will be mold-proof if it has moisture on it all the time. Even cultured marble will collect soap scum. No way to avoid it but wiping the surface down after use.