I ride the SX11 and one of my good friends rides the SX9. We both love them. They are similar but each is good for different styles of skiing. They are both predominantly for firm snow but also work fine on crud and boot-top powder. The SX11 is a stiffer ski and loves to go fast. It is not much fun at slow speeds and for short turns, but it works fantastically given a bit of speed (over 20mph - happier still over 40mph) and has no speed limit that I've been able to find. Being made for supercross events, it can be thrown around easily, is quick edge to edge and good for a bit of air.
The SX9 is easier to use at slower cruising speeds and easier to use if you don't want to charge the terrain all of the time. It still has lots of top end but doesn't want to push you all of the time. My friend much prefers it for this reason - he likes to take it a bit easier at times and doesn't want his skis trying to take off on him all of the time. When he gets to more challenging terrain, he likes to wants to more easily control his way down the hill rather than charge it and get the andrenaline running. It is harder work to do this on the SX11.
It's all "horses for courses" and so if you want an excellent ski to cruise around with the family and to give a good run at times, the SX9 would be good. If you want to spend more time at speed and skiing more aggressively, get the SX11 - you'll love it.