Cost of maintenance for a metal roof is very low. Actually if by maintenance you mean repairing and fixing leaks then the cost of maintaining a metal roof is zero dollars. The only maintenance you can do on metal roofs is removing branches, leaves, and other debris that sometimes collect on your roof and gutters. The metal color can fade after many years but it can still be repainted. Most metal roofs stays without fading for more than 30 years. As for an asphalt roof, the cost of maintenance is way more than the cost of maintaining a metal roof. Any leaves, branches, and other debris must be collected out regularly. After about a year an asphalt shingle roof should be re-caulked and sealed.
When people started installing metal roofs, there were a lot of people who used to complain that metal roofs are noisy. Some people still have this misconception but we can assure you that metal roofs are not noisy. A metal roof well installed by an experienced roofing contractor is as quiet as an asphalt roof. If a metal roof is installed over an asphalt roof then it will act as a sound barrier and your house will be quieter. As for shingle roof, they are known for being quiet.