Mike,
If I could only have one pair of Costa's, I'd go with green mirror. It's on an amber based lens and tends to be just slightly darker than the plain amber.
If I get to have two pairs, I'd go with an amber set for low light, sightfishing, etc and silver mirror for those bright bluebird days.
I believe the green mirror is a copper based lens, as is the silver mirror. Copper lenses are the best all around lens in my opinion. They can handle bright sunny days and still allow for great clearity when site fishing.
Actually, I think we're both right although it depends on the type of lenses you chose. 580's=copper, 400's=amber. My green mirrors are on 580 lenses and to my eye the copper base is a touch darker than the amber.