There are many aspects to consider when comparing asphalt and metal roofing.  Some of these are discussed below.
Durability
Metal roofs, for years now, have been hailed for their longevity. The galbanum steel which combines the strength of carbon steel and corrosion resistant aluminum and Zinc results in a powerful component that some manufactures give up to 30-50 years guarantee. The materials prevent curling, flaking, splitting, or drying out. As reported by the insurance industry, the average replacement age of say a 40 year architectural asphalt shingle is between 8-12 years because they decay in the sun as time passes thus contributing to your maintenance costs as time passes.
Cost
The cost of metal roofing, just like for asphalt shingles, is dependent on many things including geographical area size and complexity of the roof. The fact that metal roofing is actually a premium upgrade the up-front cost is more than asphalt with the rate going up to three times. The installation process of the metal roofing increases the cost. However, the long time cost from metal roofing is lower and you can save up to 60% in terms of maintenance and installation costs, a good company can install it for less money than a normal roof (see here for example)
Energy sustainability
For starters, metal roofs are well known insulators and as reported by Metal Construction Association (MCA) you can save as much as 42% on utility bills when compared to the asphalt type. In addition, this type of roofing material is recyclable thus contributing to energy conservation. The National Association of Homebuilders Research Center reports that over 20 billion pounds of asphalt shingles are dumped every year in US landfills. This means asphalt material doesn’t contribute to environmental conservation and sustainable energy practices.
Versatility in Designs and Patterns
Metal roofing accords you a chance to pick many designs shapes, colors and configurations but asphalt shingles are singular in terms of all these thus denying you a choice. 
Fireproofing
Asphalt shingles are highly combustible plus they can spread fires which a metal roofing can certainly help quell as it is fire resistant. 
Installation 
These are some of the differences but there is always a consensus that with the complicated installation of metal roofs, the cost will keep escalating. Asphalt shingles are easy to install on rafters and hence their popularity through the years. Whatever you decide to do, allow the roof contractor guide all your roofing work and assist in your choice. Metal Construction Association (MCA) you can save as much as 42% on utility bills when compared to the asphalt type. In addition, this type of roofing material is recyclable thus contributing to energy conservation. The National Association of Homebuilders Research Center reports that over 20 billion pounds of asphalt shingles are dumped every year in US landfills. This means asphalt material doesn’t contribute to environmental conservation and sustainable energy practices.
Versatility in Designs and Patterns
Metal roofing accords you a chance to pick many designs shapes, colors and configurations but asphalt shingles are singular in terms of all these thus denying you a choice. 
Fireproofing
Asphalt shingles are highly combustible plus they can spread fires which a metal roofing can certainly help quell as it is fire resistant. 
Installation 
These are merely some of the differences but there is always a consensus that with the complicated installation of metal roofs, the cost will keep escalating. Asphalt shingles are easy to install on rafters and hence their popularity through the years. Whatever you decide to do, allow the roof contractor guide all your roofing work and assist in your choice.