I have most of the MSFS versions. Am currently running FS9 and FSX.
I also have X-Plane 9.
Like all software out there, there are pros and cons about each.
FS9 has the largest number of add-ons out there. You can find many that are free-ware and are really well built.
FSX is catching up albeit at a slow pace. there are add-ons, but remember that FSX was built to run on veesta. So regardless as to how fast your XP machine may be, and how "new" your veesta machine may be, somewhere you will have to tweak something to get it to run as well as possible.
X-Plane is a different world. The graphics are awesome, but the modeling is totally different than what you are used to in the FS series. Setting up the sim is different also where the MSFS stick setup is very similar to AH, the X-Plane set up is IMHO easier. Add-ons for X-Pane, such as aircraft, are primarily payware. There really arent that many airframes that are "free".
FS2004 is still the top-dog on the porch. If for no other reason than the size of the community and all of the add-ons available.