Hi,
I have both systems but am currently using FSX, I have used flightsimulator since it started, and you do see improvements with each new version, If you are looking for the one that gives you the most addons in respect of Planes and Scenery, I would Suggest FS9, at the moment FSX is very limited to its addons in both aircraft and Scenery, but It is also a Frame rate eater unless you have a good udated computer, I am with IVAO and as I fortunately Buld my own computers, I have Kept ahead of things and get a Frame rate of 23fps in FSX. The Choice really is Yours.