Consistent exposure to salty air can absolutely cause corrosion and rust over time if the metal roof is not properly treated and maintained.
While some may point out potential concerns with metal roofs, the benefits absolutely outweigh the drawbacks for Florida homeowners.
Depending on the circumstances and the materials, either option could cost a lot of money. What you want to focus on is longevity.
Gutters are often neglected, especially on two story homes. But with the Florida rain and wind comes debris and possible damage.