You can go to American Sportbike's website and download the results of a comprehensive testing of various exhaust systems and make a more informed decision(ftp://ftp.americansportbike.com/outgoing/XB_Exhaust_Shootout_Test_Report.zip). I think that the Drummer, Spec Ops, Micron and Buell Race exhausts are all decent options for a more aggressive sound without obnoxious sound levels, but each have different sound and power curve characteristics that you may want to consider. All you're likely to get here are subjective comments. You really can't go wrong with any of the above mentioned exhausts.
The ASB test includes some sound clips to give an idea of what the exhausts sound like, but I would try to find another Bueller with the exhaust you're interested in to make a first-hand observation of tone and loudness. You can also find some video clips on YouTube of various Buell exhaust systems.