I have one bike with an old style 9 sp rear der. I'm using 9 sp barcons, which only came in the new style 9 sp ratio. Shifting is OK but not great. My 10 sp is perfect. I'm trying to hunt down a newer 9 sp rear to make it work. Short of that I'll just convert to 10 sp.
10s and 9s derailleurs are identical except for cage width. Buy a 10s derailleur and you'll be fine, even running 9s chains. If you're concerned about cage width (though you needn't be) I have 2 new in box 2002 9s Record derailleurs (carbon body, aluminum cage). PM me if interested.