Plexiglass (acrylic) and Lexan (polycarbonate) are a great alternative to glass if you are looking for something that is less easy to break. The benefits of using these types of plastics are that they are more durable, less fragile, and can be handled safely even with large sheets. Some disadvantages to using plastic are that they are easy to scratch even while performing general cleaning. They can also discolor and start to fade to a yellowish brown over time, depending on how much light it receives.
The main difference between lexan and plexiglass is that lexan is much more durable and will not break. If you need to drill holes in it, or you are using it for security purposes you would want to use Lexan as opposed to plexiglass. Lexan is also great for protecting your existing windows or storefront if they are prone to being vandalized or broken.