Borla makes, overall, the best catback system with their million mile warranty - kind of hard to beat! Moreover, they give you a nice idle rumble without being obnoxious. They do enhance performance through simply permitting your engine to breath more efficiently, However - that normally means you need to improve the aspiration of your engine at air-intake with a quality Cold Air intake system.
Where Borla or Magnaflow really sound best is teamed up with a good set of headers and, of course, that is because that is when they work best! Sound for sound's sake is not worth it but if you want performance improvement (which must include chipping your ECU so that it will not **** intake/exhaust gains) you will get the sound as well.