the one you said is the most mentioned is the best in my opinion unless of course you go without a muffler altogether because it sounds sooo good lol
As for the gains from dual.... your not gonna see that much without having some sort of forced induction, infact im pretty sure you will lose some low end torque as well. I wouldnt recommend duals unless your gonna go all out on the car because having dual exhaust on a v6 makes it seem like your gonna be make some really bad ass sound but really its going to be laughable, even with that flowmaster muffler i still think my car sounds crappy compared to GTs but it does sound good compared to stock thats for sure.
2006 V6 Pony Package Premium -Mods: 61mm Powerhouse Turbo and Intercooler, Evolution Dyno Tune(Jon Lund)
, Steeda Performance Shorty Headers. Current Numbers: 350 rwhp/420 torque @ 8.3 psi (low boost)
415 rwhp/470 torque @12 psi Car died 6/23/09
New Set Up: 4.6 3v With Manual Trans, Bolt Ons