Stainless doesn't have to be heavy gauge thus heavy in weight. My recent stainless tubing made out of wafer thin 314 military grade that I acquired from Burns stainless in California only weighed about 7 Lbs or so over my new aluminum pipes. The Mufflers are another story since the heaviest part of the system is usually the Muffs, that is why I went with aluminum.
Keep in mind that the gauge thickness also has a play on the sound decibels of a system. Too thin and you might as have open exhaust. I personally would love to be a little more quite anyway with aluminum to hide the intent of the package. My car already sounded kind of F1 with its X-tube, so no classic Mopar notes on the street.