Im a sunglass snob. Lol. I think they are among the most important pcs of equipment you can own.
I have smith, 2 pair of mj's, and 580g costas. Truth? Glass kicks plastics butt all over the place. Nobody's plastic compares to anybody's glass. Not just in clarity, but the effective polarization is far better in glass because of a lack of birefringence.
A test? Ok. Wear any pair of polarized glasses. At arms length put a different pair, the test pair, between you and a computer screen. Turn them. If the pair in your hand are plastic, light through will be blotchy., dark areas and light. If the pair in your hand are glass, it'll go uniformly black at the right angle. Stressed plastic changes polarization angles before and after the filter. High quality injection molding and frames that dont stress the lens help. The fancier plastics are clearer than straight poly but that doesnt help with the polarization issue. But glass is pretty much perfect. Just heavy.
When put side by side (glass vs glass or plastic vs plastic), I'll personally give a slight nod to MJ. But thats minor and personal preference. Glass vs. plastic is a much larger difference, and all 3 of those companies make both glass and plastic.