Answer: Photochromic lenses (commonly known as Transitions ™) are a great option for adding convenience and comfort to your prescription lenses. Photochromatic lenses use a proprietary UV sensitive technology that allows them to remain clear inside yet tint darker when outside in the sun. The difference between photochromatic lenses and sunglasses is that photochromatic lenses change between tinted and not, whereas sunglasses stay tinted all the time. The biggest health advantage to photochromatic lenses is that they are UV blocking, although most people like them for the convenience of having one pair of glasses that darken and lighten automatically when they go in and outdoors.
There are still advantages to having prescription sunglasses, as most often the prescription sunglasses will provide more protection for the eye. Most standard frames are designed to ensure you have a wide field of view and good vision, but are not focused on protection from UV light. Sunglass frames are generally larger to cover a greater area, as well as often curved to further protect and minimize exposure from UV light that would otherwise enter from the sides of the frames.
There are advantages to both photochromatic lenses and prescription sunglasses, with many factors such as comfort, convenience, fashion, and visual demands that should be considered when making a decision for one, or both options.