Try doing a search for "exhaust review" and read through some threads. Different exhausts produce different results. Some sound high-pitched and exotic, while others have a deeper tone. Decide for yourself what tone you prefer and do your research based on that. YouTube vids never do the sound justice, but they can help narrow down your choices.
The Akra is a fantastic system, great build quality, and if you go with the full Evo system, you save a bunch of weight and gain a healthy amount of power. It's expensive, but nobody seems to care how much they paid after their first post-install WOT run. The ONLY gripe I've ever read in regards to the Akra is that, for some, it's too quiet.
Check out Eisenmann and Meisterschaft as well, two other popular choices.
BMW: 2011 AW/Black E92 M3 DCT - 2010 BSM/Tan E90 335i AT
Other: 2004 AW GDB STi 6spd - 1991 Nero/Nero 348tb 5spd