The G2 has the advantages of being lower in weight, and the body is less conductive of heat if you grab it in a cold area. It is basically a plastic flashlight with an aluminum insert for electrical conductivity. Not all models can be locked out which is setting the flashlight so that it cannot come on. It has a lexan lens that can scratch and as listed here has less light output.
The 6P is a anodized aluminum which can show scratches. It is physically heavier, slightly more expensive, and has a pyrex glass lens. It is also knurled on the finish, which is like checkering, for gripping purposes.
Both run on the exact same lamp assemblies and battery combinations so they are nearly identical in that regard.
My preference is to the aluminum models as I prefer their feel, especially when screwing on and off the tailcap, bezel, or other components. However, they are not as comfortable to grap when hot or cold.