Plexiglass of more durable and from what my local supplier told me is usually used if the sign is low enough for someone to easily hit it with something because it will take a hit better. The Acrylic will resist the sun better (yellowing) but is more brittle.
That's just the nature of florescent colors.They use UV light to get that color and as a result they break down faster than other colors. I looked into it once, it's fascinating how florescent colors work.