Either is fine, but don't let some of the concerns about acrylic dissuade you from using it. You can cut acrylic, but not tempered glass (most individuals can't cut even regular glass). Acrylic will scratch, but only if it is abused. It's much harder and more resilient to scratches than polycarbonate. As long as you don't go at it 3 times a week with a dry paper towel, it will hold up for a very long time. Certain transmission ratios are much easier to find in acrylic than glass. They are also not that much more expensive, if at all, than tempered glass. If you are worried about bowing, use 1/4". I have found that for a normal sized cab, 1/8" is fine. Something bigger could benefit from the thicker.
I've had the same piece of dark acrylic on my cab for about 10 years now, with no noticeable cosmetic issues.
RandyT