If you're going true dual, it wont really matter WHAT you go with, the results will be the same.
The 4.0l v6 mustang sees gains from 10-15rwhp from just an aftermarket muffler in the single exhaust configuration. Changing out to duals honestly wont give you much more hp (1-2 at the most) if any at all. Just something to think about.
But to answer your question, i'd go with a Pypes H-pipe, midpipe, overaxle pipe system with dual Flowmaster American Thunder mufflers (the mustang GT Flowmaster A/T kit)

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Black 2005 Mustang v6, 5 speed, IUP, EUP and Leather.
Razzi 1/4 window scoops, Mustang GT Sway Bars, Flowmaster American Thunder Single Exhaust, Clear Front Turn Signal Lenses, Demolet CAI, X-cal2 91 tune, Hurst Comp/Plus Shifter, FRPP T-Lock, Ford 3.73 gears