I'm an FSXer myself, although if I started with this fantastic hobby today, I'd definitively go with X-plane...
My reasoning is rather simple:
O the amount of development going into the core product (not talking about add-ons here)
O continuous support (no such thing for FSX anymore)
O New Zealand Pro scenery (similar to VLC for FSX - accurate landclass and textures pack)- it's donationware by the way
O stability and 64bit processing
O it has a growing community with a lot of dev talents
O 3D objects and lighting (no more 2D texture trickery)
BUT....I've got about 3000 reasons why I didn't make the switch yet

Also, I'm one of those lucky ones who never had an OOM or any kind of CTD with FSX yet

But then, I work in IT and I'm a bit of maximalist, so you got the picture....
Oh, about Prepar3D...something is wishy-washy around their EULA and its future...
Dan