JMelby's post is a great summary. As someone who has also had both, I can say that the s9 is a 2013 Mercedes, whereas the s8 was the 1999 Mercedes.
Both great in their time, but I'd rather have the latest, as the difference (in improvements) is so very noticeable.
I've got to say though that I think the Respironics PR1 is also an excellent unit, but far better value for money, if you can snag one at $300 or so less than the s9.