Just want to add:
If you plan to fly helicopters as realistic as possible (far from the real thing for both sims), there isn't much of a choice. X-Plane is far superior in that field than FSX/P3D. You can use the HTR (Helicopter Total Realism) for FSX/P3D to get it a little bit closer, but it won't be enough.
But helicopters can be hard to master in X-Plane, it is almost required that you get yourself some full length cyclic.
I exclusively fly helicopters and therefor almost exclusively X-Plane, but what I miss the most from FSX/P3D is the AI ATC and dynamic AI planes, with its radio chatter.