I am 6'2" and around 190. Level 7 as well(so they tell me). I have been on both skis -- The 8+ for about 20 days so far, the 9+ during a demo. The major differences for me were that the 9+ has a heavier feel to it and obviously is stiffer. I found it to not be the most ammenable ski when it comes to short swing-type turns at slower speeds. The 8+ handles as well as slow speeds as it does when the speed picks up. The 8+ is also a bit wider with a deeper tip and makes short turns better, IMO. Although I don't have much time in moguls yet, the 8+ likely is going to be a lot easier to handle in this department.
It comes down to preference and ability. Unless you are skiing fast the vast majority of your time and really have the skills to ski effectively while going fast, the 8+ probably will be more fun in the long run. I am not an expert or an authority on ski performance, but I would wager that the 8+ is enough ski for most anyone outside of those who truly ski at high speeds and have the skills to ski with good technique in this environment.
Skiing fast and skiing fast with decent technique are two different things, really. Any ski will move fast downhill and will turn by pushing it around. If you are like me and ski fast sometimes but get leery on steeper pitches and manhandle the ski a bit, get the 8+. Because of it's stiffness, the 9+ will not take too kindly to manhandling the skis or mangling technique. It likes proper edge sets and doesn't like to be forcefully pushed.
I thought both skis are relatively sensititve to fore-aft balance and are not quite as forgiving as a ski like the RX8.