Where as i have found, X-Plane is more about real world dynamics, the planes seem to act more real, but even though you can get 15GB of scencery for it, the "Game Play" or the whole environment just doesnt feel realistic. I have no add-ons for X-Plane, i know there are a fair few, some maybe these addons make the experience seem more real?
Depends what you're after. X-Plane has a very active community. Personally I use both FS and X-Plane, but I am increasingly leaning toward the latter - it is catching up very rapidly indeed.
A strength of X-Plane (other than the lovely flight model) is that all simulated data can be outputted - you can see what the flight model is generating both numerically and graphically. Much of this data can be written to as well. There is an SDK interface that has allowed some great mods, including a superb fly-by-wire script, a pushback tug, a traffic 'recorder' (allows you to create flights at your airport of choice, and have them played back to create activity at your airport, outside of the AI controlled planes).
For my money the global scenery package is on a par with MSFS, it's very good indeed. X-plane's aircraft were originally limited by the built in planemaker. The more inventive modders have created some truly stunning aircraft, but now Planemaker supports the obj8 format, so it is possible to have high-detail, animated models that are aliased to the planemaker theoretical model (necessary for X-Plane to calculate the flight dynamics).
The fact that the instruments are silky smooth is a huge plus if you're into IFR. There's a demo if anyone wishes to try it.
Edit: Weather simulation is also excellent.