just to make your choice more confusing, i too, just bought the costa 580 harpoon in the green mirror. The mirror is really the same as the amber lens 580, but it has a coating on it that supposedly reflects a bit more glare. I called costa and told them i wanted the "ultimate bonefishing glasses" and they said to go with the green mirror 580.
one of the things to consider is that plastic lenses are significantly lighter, but i've found the glass to be much more scratch resistant. i have two pair of costas and my 3 year old picked my 400's off my hat and tossed them on the concrete at the pool this summer, and no scratches.