Hi guys,
I use to work in the exhaust industry, and I can say that if you start your car up and run it for less than 5 minutes, all your doing is gathering moisture in the rear can of a complete exhaust system. (as was said above)
Given nowadays that most new systems on cars are full stainless, and tend to last >7 years, stainless is the better for this application.
Now given what this website is all about, and when we do get into the cars we tend to either give a quick blast on the throttle to impress our mates who are over for a beer and barbie, the moitsure is blown out of the system. I had a mild steel set of extractor on my old VY SS that did the trick and an alumised steel exhaust system for the rest of it. I had the car 5 years, and never had a problem with rusting, as it was given a caning most of the time, as it sounded great
, and went to 6250rpm as quick as.
Given the cost factor, (Stainless steel is usually 2x the price as it has a higher wear factor on the tooling used to bend the pipes, but will last 3x as long on your wifes car), I will be going for a normal alumised steel exhaust system on the car, with mild steel extractors. I have not got as far as worrying about coating/painting yet.
You can put a small hole in the lowest point of a muffler, as Walker do on some of their mufflers, but all your doing is breaking the coating over the mild steel and giving rust a nice place to start. Usually the muffler will need to be replaced, and very rarely a set of extractors unless you hit something.
Stainless steel extractors tend to crack more than mild steel extractors, due to the combination of heat and the weight of the system flexing at some point.
If all else fails, just drive up a long hill towing a caravan, then your system will get hot the full length of the exhaust system and will evapourate any moisture, or if that sounds too volvoish with tweed hatish, just rev the crap out of it when you can.
Hope this helps.