pretty much as a rule, the more free flow an exhaust is, the louder the exhaust is going to be. Free flow generally equals performance, however in most engines you do want a little back pressure (unless its a turbo.) Dont be overly concerned though, because back pressure shouldnt be an issue on a "normal" diameter exhaust system with mufflers.
dont quote me on this, but the shorter the exhaust pipes + greater diameter = louder. Mufflers help mellow everything out. Catalitic converters will also help make it quieter, but eat up performance. As for the mufflers real quick, most brands have a particular "sound" that characterizes them. check with guys who have exhaust work on their cars and see what you think!
I live in the nevada high desert, and here its dry as can be. But i still went with stainless on my daily driver because its less prone to burning out like other metals will. Stainless will basically last you forever.
sorry if this was a little random /ubbthreads/images/graemlins/crazyeyes.gif, i just woke up /ubbthreads/images/graemlins/smile.gif