Some other factors to consider are steer clear of sunnies that affect your peripheral vision i.e. sunglasses with thick plastic arms.
Don't buy sunglasses that are tinted too dark (you want to be able to wear them in dim light as well to reduce glare).
Sunglasses with big thick arms or ones that jut out like some of the old Oakleys did just wont fit, or will be uncomfortable underneath a GA headset.
Serengeti's are great as the photo chromatic lens gets darker as the light intensity increases. But they are expensive ~$350. Sunglass Hut and Bright Eyes both stock Serengeti's last time I checked.