As was said Plexiglass is a brand name like Kleenex, it's an acrylic.
Lexan is actually also just a brand name, it's a polycarbonate.
Acrylic = Plexiglas, Perspex, Optiplex, Lucite(what I use), etc.
Acrylic is more resistant to scratches and is therefore the obvious choice for covering a CP. The only disadvantage is that it can crack easier when you're drilling it.