Acrylic is a fairly new material for making aquariums, compared to glass. It is lightweight and is much harder to break than glass, although it is easier to scratch. Unlike with glass, the scratches can be easily removed by buffering the acrylic.
Acrylic is easily molded and formed into almost any shape. Acrylic also has less of a tendency to distort things that are behind a curve. Acrylic has nearly the same index of refraction as water. Because of this, the only distortion you are likely to see is that the fish is slightly misplaced, but the size and color are true.
If you are setting up a large tank - 300 gallons or more - or you are interested in a tank that is not rectangular in shape, you will probably want to consider acrylic.