Perspex would be best. It's lighter, easy to cut with a jigsaw, and won't end up in a thousand pieces if you drop it
I agree with Bursar Although, my window is made out of acrylic (They say), so I dunno the diff between acrylic and perspex... but I'm going to make the protective plate for my LCD out of Perspex
Acrylic - More Exspensive, Stronger, Can be bullet proof depending on thickness Perspex - Cheap, Not as strong It could be the other way round tho