Lifespan is the first thing people ask about, and it’s the least useful number for deciding between the two. A well-installed architectural shingle roof and a well-installed standing-seam metal roof will both outlast most owners’ time in the house. The decision usually comes down to three other factors: upfront cost, roof pitch, and how long you plan to stay.
Metal costs more upfront but needs almost no maintenance and handles steep pitches and heavy rain better. Shingle costs less upfront, is easier to repair in sections, and is the better choice if you’re not planning to stay in the house for the roof’s full service life. Neither is the “better” roof in the abstract — they’re suited to different situations.