Plexiglass is acrylic (PMMA), and Lexan is the trade name for Polycarbonate (PC) made by GE (actually Sabic now)- same thing as Makrolon, which is made by Bayer.
Acrylic is less shatter resistant, but it's easy to buff scratches out of (use turtle wax). It's other main benefit is that it's UV stable- means that it won't turn yellow with age.
PC will turn yellow pretty quickly when exposed to sun light if it doesn't have a protective coating.
For cost/ lb, acrylic should be about 2/3 the price of PC.

And I never thought my job would come in handy when talking about snakes.....