Have the 6's. Great speakers. I've heard the 9's in a different room with different gear (Thanks Sunil!

). If I had the room and the money, there is no question I'd get the 9's. They are truly fantastic speakers. As Danny said, more dynamic and incredible low end. Pair them with good source and power, you will be a truly happy camper.
I've heard better speakers, but we're talking serious, serious money.
(Those LS-X's.........sigh)