Question--are you photosensitive? If so you will definitely want something that can get dark, be it a magnetic/spring-loaded sunglasses attachment or the Transitions® (yes, that's a brand name) lenses. Check around with people who have bought them from whatever optical you are going to, I've heard some are better than others.

I am very photosensitive and normally wear sunglasses all the time when I'm outside unless it's really dark out. They help you see better and also prevent macular degeneration.