Based only on what I've read, certain brands have better strength. I can't say whether that's actually true mind you.
Second paragraph:
http://www.associatedplastics.com/acrylic.php
I've always read good things about Acrylite GP.
Edit:
A ton of my reading was regarding Envision Acrylics. I was close to ordering a tank from James a really long time ago.
http://www.envisionacrylics.com/aquariums.html
This guy seemed to know the material VERY well. And he always used GP.