to put it short...
standard/extended Exhaust has a sound level that is legal in every state. Besides having a legal decibel rating, it has a sexy low growl in the low-to-mid rpm range. and when in the higher rev range, it screams like a hott chick having an orgasm
extended/standard exhaust gives the highest HP gains out of all kits that can still maintain a legal sound decibel.
Now if you are not into the soft tones, you might want to check out the TRD
Kazuma, which gives the highest gains - even higher than the TRD
extended/standard exhaust kits. but beware, the TRD
Kazuma is not, NOT, street legal. It is too loud and can be heard from far distances. A sales ad for the Kazuma states that it is "For Off-Road Use Only." But if you decide on buying and installing it, dont bitch about getting tickets.