Acrylic aquariums may weigh less than glass, but don't let that fool you. Acrylic is a very strong material, and it takes significant force to break an acrylic aquarium, and anyone who has every owned a glass aquarium can tell you about how even little bumps can cause cracks in the aquarium and cause it to be structurally unstable.
Even though glass is thicker than acrylic, it is much weaker. Acrylic aquariums are very resistant when it comes to bumps, however they are more easily scratched than glass aquariums. When it comes to customizing the shape of your aquarium, you are pretty limited when it comes to the glass aquarium. Glass is relatively brittle and rigid naturally, and because of this, its shapes and styles are very limited. Acrylic aquariums on the other hand, can be molded to fit any shape and design that you could wish for.
Acrylic aquariums can be cast and molded into different shapes that were never though possible when only working with materials such as glass. Acrylic aquariums are also much easier to cut holes into when attaching an overflow system, with glass you need specialized tools and then you run the risk of cracking the whole panel.