Plexiglass/Acrylic is the preferred material for your application. The polycarbonate/Lexan is stronger, but it is subject to yellowing. Scratches can be buffed out of plexiglass. Plexiglass has better optical clairity than polycorbonate (not very important for your application.
There are different qualities of plexiglass, so be sure to ask before you purchase the material.
Select Plastics has a FAQ section that explains the difference and the pros/cons.