I'm a sunglasses snob. I admit it. I consider it perhaps the most important piece of equipment for a fly fisherman. Not just for seeing fish but reading water and structure. I'll spend $250 on glasses and use a $100 rod/reel combo in a heartbeat over the reverse.
Brand aside, there's a very large difference between stamped and injection molded. Stamped are the $10-$20 cheapies you see at gas stations, Walmart, and the like. The stamping process induces a lot of stress to the polarizer and effectively makes it far less efficient. Injection molded sunglasses start at around $40 and go up to over $200, and includes the lower end offerings of all the big sunglass names like Maui Jim, Smith, Costa, Oakley, etc. as well as the main line of your Sunclouds, Natives, and the like. There's a million brands in this category. If it's over $40, it's probably injection molded.
But I won't lie, you can get a perfectly serviceable pair of sunglasses at about $40-$50 that will be very nearly as as good as many costing 4x that much. Lot of marketing fluff.
To take the next step, it's about materials. The $40-$100 range (and Oakley, lol) is polycarb, the stuff safety glasses are made from. Tough, but not very clear. There's all kind of proprietary plastics out there with better clarity than standard polycarbonate. Your eye doctor probably calls them all Trivex, lol. It's the stuff regular eyeglasses are made from, and each brand has it's slight tweak to the composition. These too are injection molded and will make up the upper end of that $100-$200 range of injection molded plastics. The polarization isn't any better. Just a touch more sharpness on distance vision. Which can be important when trying to spot a tail in a stream. But certainly not a night and day difference over injection molded polycarbs that typically sell for much less.
The last step up is glass. It's a rare bird these days but the optical clarity is second to none. And there's a further advantage for fishermen. Even injection molding gives some stress to the polarizer. Not as much as stamping, but some. Glass gives none. It proves itself out with polarization tests, where glass lenses outperform all plastics. And the effect on the stream is noticeable. So in addition to having the best clarity glass lenses have the best polarization as well. They also have better scratch and heat resistance, so they hold up better over time (provided you don't drop them or sit on them). And they make a better substrate for coatings such as anti-reflective and mirror coatings, so they don't wear off as easy like some plastic coatings do.
But. To get a glass lens, you are talking $200 and up, and not everyone is prepared for that for what amounts to a modest upgrade in performance. There's only a couple of brands that offer it. Maui Jim (SuperThin Glass), Costa (400G and 580G, the g stands for glass), and Smith (Techlite Glass) for sure. Ray Ban and Serengeti used to but I'm not sure if they still do. And glass has a few drawbacks as well, which frankly don't bother me. It is heavier, so be fussier about fit. All glass options are full framed, none of that rimless stuff. And it can shatter, they should not be considered safety glasses by any means.
IMO, if you're going plastic, stay in the $40-$70 range. If you are going to go premium, for a fishermen I'd recommend skipping the fancier plastics in the $150+ range and go straight to glass in the $200-$250 range. That leaves 3 tiers.
1. Cheapo stamped polycarb - $10-$30
2. Injection molded polycarb - $40-$70
3. Glass - $200+
Each being a fairly significant upgrade over the level prior. I'm a Maui Jim fan but I won't knock Costa or Smith glass options. All are excellent products, it's about preference of color options and style at that point. Costa's tend to be too dark for forested PA, IMO, and they have a low light option but don't make it in glass. Awesome for saltwater guys, or out west, though. All 3 do some fancy color enhancement stuff that all work about equally. MJ does have better anti-reflective coatings on the inside, but that only matters if you have a poor fitting pair that lets light in behind, it doesn't matter if the fit is good.