I'm using both. If you like an immersive environment then yes, FSX will be best for you, what with the busy AI traffic and realistic ATC system. X-plane falls way behind in these areas and requires you to download scenery for any airports.
I use X-Plane to design aircraft and fly them, because it is far better and more realistic for that than FSX; its flight modelling system allows you to "predict" characteristics of a fictional aircraft.
No idea about this P3D simulator.