custom you have some control ova. every1 will agree its a matter of where you have it done. i went custom mostly since my car is low and i wanted additional low end power. Catbacks are designed to give up low RPM gains for High RPM preformance. where as Axle backs give the power to the lower RPM's at the risk of High RPM gains. the only difference is with axle backs you have to upgrade your piping (in most applications). for me i got aftermarket piping and everything just the last shop i went to wanted to be lazy. i got the preformance i wanted out of it plus a sound that i wanted (lower tone but not as loud as my previous exhaust). my only complaint is how they did my piping instead of fitting it into the trench they just mounted teh piping below my cross members. i could just redo the piping instead but for me it's cheaper and faster to just get a cat back and mount it myself (no welding needed)