How large is the tank. Very large tanks usually have to be made of acrylic.
Acrylic is lighter and very clear, but scratches easier and will bow slightly, even if built properly. It will bow allot of built thinly. Although it does scratch easier, it can be repaired, but does require the tank be drained. Glass is heavier, doesn't bow, isn't as clear, and doesn't scratch as easy. If proper practices are used, you should rarely scratch a glass tank. However, if it is scratched it is near impossible to fix. Pluses and minuses for each.