I have had a chance to compare the 580 glass, 580 poly, and 400 glass. I'll share what I have learned on here. Remember, This is only matter of opinion.
My first pair of Costas were a 580 glass green mirror pair and I was just blown away by them. I couldnt get myself to go outside without them. I could spot flounder, and reds where other people couldnt see below the surface of the water.
I decided to get a backup pair of 580p Costas and I must say that I wasnt very impressed with them out on the water. In certain lighting settings I could see as good as the glass lenses, but most times I couldnt.
I then picked up a 400 glass pair with the same green mirror. They are much clearer than the 580ps, and almost as clear as the 580 glass. The difference is so feint that I dont think it warrants the extra $50. I do, however, believe that the extra $40 from 580p to the 400g is definitely worth the price.
I decided to post this because I did alot of online searches and I never found any forum compare the 400 glass to the 580p. Please chime in if you have any thoughts or opinions on the different lenses.
EDIT : During my research, I did find that Offshore Anglers noticed a difference between the 400s and the 580s ,where as, inshore anglers tend not to notice at all or very slightly like my experience