Metal (steel) roof panels are much stronger and more durable than Aluminium.
Either way you go, both are subject to salt water corrosion. And both are subject to expansion/contraction shifting and leaks.
My strong recommendation, is that you find a local contractor (at website provided) and have your new roof sealed and protected with a waterproof membrane, and reflective ceramic coatings.
I highly recommend two companies; www.icc-astec.com and www.supertherm.net
I have used "Astec" products with nothing short of phenominal results. Please see one of my jobs at www.energysaviours.com and click on "about us". This is my own website, and is still under construction, so please forgive the lack of content, I'm working on it.
Best of luck with whatever you choose.
Regards, Brian (tankless) Wood.