Stainless Steel

Most aftermarket exhaust systems are stainless steel.  Stainless steel has become the standard material used by exhaust manufacturers because of its durability and relatively low cost to produce.   Most consumers never consider anything other than stainless steel when they decide on their aftermarket catback exhaust because it is the most cost effective.  On average, a typical stainless steel aftermarket catback exhaust system will save approximately 20-40% in weight over the factory catback exhaust system.

Titanium

Titanium is another type of material used in the construction of aftermarket catback exhaust systems.  Titanium is extremely strong and lightweight.  
Typically, a catback exhaust constructed out of titanium tubing and fittings will save 50-60% in weight over the factory catback exhaust.  Exhausts made from Titanium also offer a sportier look and exhaust note than their stainless steel counterparts.   However, the major drawback is cost.  These exhausts tend to be more expensive and as a result are not manufactured in the same numbers, so availability at any given time can be hit or miss.