Most pre-fabricated kits (RK Sport, Magnaflow, etc) are quite expensive, and this is usually the deciding factor for most tuners. Any good exhaust shop can build a quality system for less money, however you usually have to settle for aluminized steel pipe instead of stainless steel. In the end, you generally get what you pay for.
Is one better than the other? Well, no one has ever dyno-tuned an exhaust system to say for sure that one system out there is "the best", so take caution when someone claims their system is "the best". There are many pre-fabricated systems out there, find the one that fits your goals and budget and go from there.
