Well i have had FS since 98, and i think its more about the game play and giving more real world feeling to the whole game,
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?
There is also FlightGear flight sim, has a few claims to fame as NASA, BAe, M-D, Boeing, Airbus and for sure more have used it, it pairs very nice with a few packages such at MAT-LAB/SIMULINK...
I am the only one who might well think this, But FS-X to me is a pure game, i cant take it serious, i think its because of all the things "games" you can do, like dropping em packages on targets and doing the red-bull air-race. I havnt had a serious flight on it to date...