Dual-Mode Performance Exhaust v. Cat-Back Exhaust

Hi All:
I'm getting ready to order a 2008 Convertible with paddle shift automatic and am considering ordering either the Dual-Mode Performance Exhaust option
(6hp gain), or the dealer installed Cat-Back Exhaust option. Can anyone give me some info. as to which one is a better option? Which one may give a better sound? Which one may give a larger horsepower gain?
Thanks, Scott

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