In general, glass is cheaper to purchase than plexiglass, is more scratch resistant and more easily recycled. Plexiglass, on the other hand, is stronger, more shatter-resistant and resistant to the elements and erosion than glass.
Plexiglass is a type of clear acrylic sheet. The scientific name is poly(methyl methacrylate), sometimes abbreviated as PMMA and classified as a transparent thermoplastic. Since plexiglass is a plastic, it is a petroleum-based product. Glass is a amorphous (non-crystalline) solid, an inorganic compound that has cooled from a liquid to a solid without passing through a crystalline state. Glass is brittle, transparent and composed primarily of silicas.
Glass surfaces reflect light more readily than plexiglass surfaces, and this can create unwanted glare or reflections. Many aquarium makers use plexiglass not only for its greater relative strength but also because it offers less glare and increased clarity.
Standard glass is cheaper than clear acrylic sheets. However, prices will depend not just on size and thickness, but also on added features such as UV-cut and glare-reduction coatings. In general, when comparing sheets of glass and plexiglass offering the same benefits, the glass option will be cheaper. However, given that plexiglass is more resistant to weathering and erosion than glass, long-term maintenance and replacement costs can become significantly cheaper with plexiglass.
Shatter and Scratch Resistance
Glass is an amorphous material, a "frozen" liquid, and thus much more easily shattered than a thermoplastic such as plexiglass, which is softer than glass. One problem with plexiglass is that it is subject to crack formation when it is brought into contact with common solvents. This makes cleaning plexiglass more difficult as you must choose cleaners that are safe to use on it. Improper cleaning can scratch plexiglass. In general, glass is more difficult to scratch than plexiglass but more easily shattered.
Glass has about twice the density of plexiglass. Therefore, a sheet of plexiglass weighs less than a sheet of glass of the same dimensions, and this presents many added benefits including easier handling and shipping costs.
Clear acrylic sheets are much easier to cut than glass sheets. Plexiglass can be cut a wide variety of shapes to fit the user's needs.
Glass can be recycled cheaply numerous times. Plexiglass (since it involves the use of petrochemicals) is more difficult, and therefore more expensive, to recycle.