The level the sub peaks is a very clear indicator of the general loudness of a track, most of that headroom is thrown into sub when that -3 is present.
Some examples of -9:
http://soundcloud.com/owslaofficial/kill-the-noise-feed-me-t humbs-up
http://www.youtube.com/watch?v=dasOi4Fsy6Q
Some examples of -6:
http://www.youtube.com/watch?v=gcejLp72iCE
http://www.youtube.com/watch?v=he2jtqnSj-Y
Do you like one or the other more? Does that extra boost in headroom cost anything/worth it?
On average -6 mastering sounds like its got that extra oomph to me.
this whole thread is obviously aimed towrards aggressive edm, electro house, dnb, etc