Metal roofing is somewhat more energy efficient than other roofing types, due to the fact that light colors are available. Unlike a dark-colored roof, which absorbs solar heat into your attic, a light colored metal roof tends to reflect more of that heat back into the atmosphere, causing your air conditioning to run a bit less in the summer. While significant, this benefit alone is not enough to be the sole deciding factor, especially if a light colored roof does not complement the exterior of your house.
The best metal roofs last 50 years or more, a period of time in which a shingle roof, for instance, would have to be completely replaced at least two or three times. The tear-off from such replacements often finds its way to landfills; hence, metal helps the environment by reducing waste material.