Well, I've heard the Magnaflow muffler sounds fairly good - only problem with that system is that it includes another set of midpipes with a resonator.
Most who add a performance muffler also do a resonator delete to make the sound even better. Even the BMW Perfomance Exhaust includes midpipes WITOUT a resonator.
Seeing as you already have a resonator in your factory system, if you want to keep one - you'd be better off just buying the appropriate Magnaflow muffler only and custom welding it onto your existing pipe, instead of paying for yet another resonator.
There are plenty of threads showing how the muffler alone installs if you do a search...
2009 328ix E92//SGM//ACS Roof Spoiler//MTech Aero Lip+Trunk Spoiler//M3 Side Skirts//CF Roundels+Grilles//M3 Novillo Interior//Perf Shift Lever+Boot//MPerf DCT Wheel//M3 GT3 eBrake Handle+Boot//ACS Pedals//M3 Mirrors//LCI Tails w/LUX Rev//DTEC V7 AE+MTec 6000K D1S+HPB HID 6000K Fogs//HPB LED Interior//RPI Scoops//CF Euro Airbox+Underhood Hsgs//K&N//BMW PE//MTech Rear Bumper-iS Sport Diffuser//CF Strut Brace//Vishnu OFT//BMW 193M's+Kumho 4X