I've worked in a number of fly shops and sporting goods stores that sold both. Both are high quality and both have been in the business a long while. Both make glass lens, or have them made for them, both have high tech lens other than glass. Both hit the advertising market hard but Costas gets a point or two on a more artsy fartsy mode of ads. In the end analysis my bet is what you hear in most cases is based on anecdotal evidence as any can lose a screw, etc and far too often folks make up their mind about what's best for them without really looking into the facts. Smith started back in 65, Costas in 85 and Action Optics, a brand of Smith's in about 92 and all do one thing well and that's
provide a mirror at the point of sale display so the person can admire the image of them and their glasses. For some it's frame color, others opt for style, and some go for the ads they read, heard or seen but few really get into the nitty gritty, and really don't need to, since in this case they both have a great grasp on the "nitty gritty."
As far as mirrored finishes,
the nod goes to Ray Ban who pioneered that back in the 60s, possibly before and who, by the way, has a superior product also.
The mirror works to and by reflecting sunlight away from the eyes. My favorite for water work (years as an angler, SCUBA and Swimming Instructor) was the double gradient lens with heavy mirror top and bottom knocking back the sunlight plus the reflected sunlight coming up from the water.
When it comes to lens shade, I like a couple variations, one fairly dark and one very light for morning and overcast day fishing. My lights are ultra light yellow Hobies another top notch glass. Of course good polarization is a must.
Speaking of lens color, it's a tad bit complicated but here's several forum posts that cover it pretty much from stem to stern. Best sunglasses lens color for streams...
Read the first string, then look at the bottom for more posts on the subject.
My choices are
light yellow for contrast under low light conditions, Copperish brown for flats fishing, green for mountain/stream fishing and were I a deep water fisherman, a darker blue. Your choice of lens color, or tint, depends on several factors but mainly the color of the fish, the color of the bottom and the colors reflected into the water from surroundings.