Get the normal organic aka plastic. Polycarbonate aka Airwear Transitions effect not as good as normal organic due to constraint on the material. Poly is thinner and lighter with 1.6 index though. Normal plastic go as high as 1.74 but tat gonna cost you a bomb if your power is high.
You cnot convert an existing sunglasses lens to transitions. I think the photochromic material is added during the molding process. However, you can replaced your sunglasses lens with prescribe transitions lens. Provided your frame isn't those wrap around type. Due to the curvature, not much labs can edge curve, prescribe lenses. Even if they can, there will be distortion.
Although it can block out almost all uv, transitions lens are not as good as Oakley in term of blocking visible light or glare.
Transitions work with UV. If your car had uv protection coating installed, don't buy a transitions lens solely for driving. Never work coz all the uv had being filtered off. This is quite important as friend of mine jump up and down over this after spending close to 1k frame plus lenses. Get a polariser lens instead.
Hope this helps.