De cat A

De cat B

A seems to be the most common one, it replaces the cat. But B replaces the middle box as well as the cat (on a standard exhaust) ??
If so which is better? is B replacing the middle box for sound reasons or power?
Whats the difference? I can only think B is designed more for sound than power due to replacing the middle box?
De cat B
A seems to be the most common one, it replaces the cat. But B replaces the middle box as well as the cat (on a standard exhaust) ??
If so which is better? is B replacing the middle box for sound reasons or power?
Whats the difference? I can only think B is designed more for sound than power due to replacing the middle box?