Gordon,

Plexiglass is an acrylic plastic. Plastics yellow when exposed to ultraviolet light unless protected by using a UV inhibitor in the manufactue of the product, however not all acrylics contain UV protection. You can't tell just from looking at it, you need information from the manufacturer.

Even with protection, plastics may evntually visually degrade when exposed to weathering because they scrath easier than glass and even simple cleaning may eventually cause the material to become hazy due to scratches from windborne sand and dust as well as cleaning them.

Personally, I would use glass.