Metal roofing is fire resistant and you may get a discount on your home insurance. There is less chance of leaks and water damage. It is also a good choice environmentally. Metal roofing is partially composed of recycled steel and it can be recycled again in the future. Asphalt shingles are probably the worst choice because of their short life span and quickly end up in landfills.
Major savings come from using white or light colored roofs to reflect heat away in hot climates.
A properly designed and well insulated home will not need a swamp cooler or air conditioning.