There is no significant difference in sound or performance based only on metal type, but in general you get what you pay for with exhaust systems. 304 stainless is the marque of high end systems such as Borla or Corsa. You will find a superior fit and finish with these systems even if corrosion resistance is not important in your area. Also the engineering will be superior....pipe diameter is selected to give a performance boost without compromising low end torque with the "bigger is better" syndrome.
Great info guys. I figured the SS would be the best bet, just curious if it would be worth the price difference for it...I guess I would rather have stainless and not need it; then need stainless and not have it..