Of all that I've heard, Borla and FRPP Sport sound the best, BUT only a sucker pays retail. I went with Magnaflow Comps because I got them for $200. Get what you like, but check for used - you can get a deal.
2011 Mustang 3.7L, 6A, MCA Pkg, Ingot Silver w/ Stone
Perf Mods: FRPP 3.73 Gears, 87 Octane Performance Tune from BAMA, Airaid CAI, Magnaflow Comp Axle-backs
Ext Mods: Lower chrome billet grille, Tint
Upcoming Mods: Lowering springs