Cost
Asphalt shingles installed run roughly $5–$8 per square foot in our area. Exposed-fastener steel roofing is about $9–$14, and standing seam (hidden fasteners, the clean modern look) is $14–$22. On an average home that's $12,000–$20,000 for shingles against $25,000–$50,000 for metal.
How long they last
Shingles around Georgian Bay last 15–25 years. The wind off the bay and the freeze-thaw cycles are hard on them, and the cheaper 3-tab shingles are at the low end of that. A steel roof lasts 50 years or more. The paint finish usually carries a 40-year warranty. So over 50 years you'd replace shingles two or three times, and each time the price is higher than the last.
Snow
This is the big one up here. Metal sheds snow. That's good for the roof structure and bad for whatever is underneath when it lets go — so we install snow guards over doors, walkways, decks and lower roofs. Shingles hold snow, which means more ice damming at the eaves if your attic ventilation isn't right.
Noise
A metal roof over a solid deck with underlayment is not loud. The "rain on a tin roof" thing is a barn with no deck underneath. Inside a normal house you won't notice a difference.
Over the old shingles or not?
Often we can go over one layer of shingles with strapping, which saves the tear-off and disposal cost. We check the deck first. If it's soft, or there are already two layers, the old roof has to come off.
Resale
Metal helps resale on cottages, rural homes and anything a buyer sees as "low maintenance." On a subdivision house in Barrie it's less of a selling point, but it still removes the "roof needs replacing" conversation from the sale.
Our honest take
Cottage, rural property, big snow load, or you plan to be there 15+ years: metal. House you'll sell in under five years: shingles. We do both, so there's no reason for us to push you either way.