Oh and as for using SS: its heavier, retains heat a little better so can help reduce noise and maintain flow through temperature but don't think SS will last much longer. Aluminised  MS will provide a similar level of resistance to corrosion. Corrosion in exhaust isn't caused by water but rather the exhaust itself. Nitric oxides and mixed with water, nitric acid, is one of the main causes. SS is not resistant to nitrogen oxides or nitric acid and can corrode at rates as high as 1mmPA.
 
Personally (and from on my old mans advice) SS shouldn't be used for its marginally better corrosive resistance as you'll never get the value out of the much higher cost. Get it because you need a heavy muffler, or heavy exhaust to stop resinating or because you like the look, but it wont last more than 1/3 longer and will cost you three times as much.