Depends on your view. X9 has more features with mods and such, but X10 looks better, runs better, and overall in my opinion is better. The NPCs help you or destroy you. Or they can just stand around. AI is good, but can be a bit buggy if you have a lot of entities around your level as they don't want to step on entities. You can create in-depth storylines with sound zones, trigger zones, loading screens, and video zones.
My 2 cents.
http://forum.thegamecreators.com/?m=forum_view&t=166194&b=25
RADS, X10 Game. Still in Development as of 4/6/10