This is one of the best performance upgrades you can invest in, so spend your green wisely.   It is something you will probably not do again so get it right the first time.   You are looking for a system that is made of stainless steel with mandrel bent tubing (keeps its shape throughout the bends with no flow restrictions-no kinks) and carefully selected pipe size (bigger is not always better).  Each system should be fit checked and dyno tested to verify performance.   Make sure the system does not sacrifice low end torque for top end horse power.   Consider the manufacturer.   Are they nationally and/or internationally known?  Is the company you are buying the system from an authorized distributor and/or retailer that has the ability to warranty the product?  
MagnaFlow Exhaust not only fills all these requirements, they have the best warranty around.  Their research and development center does not release a cat-back system until maximum horse power, torque and sound quality is achieved.   Greyhound Performance is an authorized distributor for all of their products.