I have an acrylic shower that is almost 20 years old (biscuit colored) and an acrylic tub/shower combo that is over 25 years old (harvest gold - to be replaced next year),

They both look as good as new, except for one small spot around the drain of the tub where I spilled a little bit of drain cleaner and it crackled very slightly.

I will be replacing the harvest gold one with a white when I redo that bathroom - not because it is worn, but because it is gold :).

In my experience using other people's showers, fiberglass feels thinner, flimsier, flexible. I don't feel that at all with my acrylic. And they are SO EASY to clean!

Go with acrylic - they are too difficult to replace.

Others will tell you to tile, and I love beautiful tiled showers, but I just can't deal with grout.

Acrylic is the better material. I don't have fiberglass but I've been in plenty fiberglass tubs and showers. I wouldn't have it in my house if I could help it. The thing about acrylic is you have to be very careful what you choose to clean it. You can't use abrasive products. Believe it or not, Scrubbing Bubbles is "abrasive". The best product I've found for acrylic is Gel Gloss. I get it at Lowes. Time consuming as it calls for an extra step. You have to buff it to a shine. But the shower will literally glow! :) Stays cleaner longer too.