Lexan makes more sense for a control panel than for a monitor shield. Unless you plan on having some ape punching your machine, you might as well get the glass. It doesn't scratch like synthetics, it doesn't dull, and it's cheaper.
I had plexi on mine, though, and it's a good thing I did. A BIG friend of mine got mad at the game and punched the screen. He shook the whole machine. Yeah I nearly bum rushed him until it occured to me that he was twice my weight. Still, he didn't play anymore after that. Ah, the joys of having a machine that lives in a frat house...