Why not the cheaper, more easily available stuff? The Stained Glass Overlay or the film coated variety?

The answer is in reflection and refraction, Answer is in a real painting and a printed poster. The best description of glass, to my mind, was in the book "How to work in Stained Glass" by Anita and Seymour Isenberg:

"No defination of glass is satisfactory in terms of describing all the permutations of this remarkable material. Physically it is a paradox, chemically it is idiosyncratic, artistically it is challenging, pragmatically it is ubiquitous, prismatically it is spectacular, ocularly it is both far and near seeing."

Glass is classified as a liquid. Yes, even though it seems solid, it is a liquid in a supercooled state. Its molecules are in a permanent state of disarray, keeping the light reflecting and refracting in its most dazzling form. Nothing compares to a piece of genuine glass, Some of the best glass comes from America and Germany. Spectrum makes a variety of colored glass, Armstrong and Wismach make some brilliant combinations of colors. We use glasses from all these companies.

Some of the Stained Glass panels made centuries ago still survive in some old cathedrals and churches. The oldest known was made around 900AD.

If you are looking for a real piece of Stained Glass work, something that will not fade with time, not lose its brilliance, come to us.