This is from the Metal Roofing Alliance web site:
Q. Would a metal roof be too heavy for certain types of homes, or for smaller structures like a detached garage or porch?
A. You'll be surprised to learn that a metal roof is, on average, 50% lighter than an asphalt shingle roof, and 75% lighter than concrete tile, fiber cement shakes and slate. With metal roofing, weight on a structure is never an issue. (The above comment is on this page.)
This is from the Asphalt Roofing Manufacturers Association web site:
Asphalt steep-slope roofing products are designed for safe, speedy and hassle-free installation all year round in any conditions. Weighing substantially less than rival products such as tile, concrete, slate, wood, and metal, asphalt is easy to store, transport and unload. (The above comment is on this page.)
So, who is telling the truth?