The XXX handles more power, and has more displacement potential. I realize the XXX loses a bit due to efficiency (thanks to XBL^2), but Id be interested to hear why people think the HC would be louder, and how they came to that conclusion.
And I understand where you are coming from jemorgan, but using 'we all like different things' to try to say a driver that clearly outputs more distortion (the HC) sounds 'better' isnt really fair. Its true some people find the amazingly low distortion output of XBL^2 enabled drivers a little weird, because we get use to the distortion inherant in any non-BL optimized setup, but the truth is the driver is actually reproducing the sound more accurately. IMO that should be the subjective definition of what SQ is, not what each person's personal preference is. Many things in SQ cant be defined scientifically (width and depth of the soundstage, realism of vocals, etc) , but this is one of the few things that can: distortion output. All other things being equal, the XXX outputs less distortion than does the HC. And this difference in distortion increases as cone excursion increases (the XXX winning out). But please do not take this as a knock on your personal preference.
Lastly, there have been quite a few threads on the boards, is the SX louder than the XXX, does it sound better, etc. Now some people are even saying the HC beats the XXX. I think there is a good reason RE charges twice as much for a XXX as it does the HC, XBL^2 and higher excursion (on the XXX) aren't just marketing gimmicks.