As Pieces said, I think you'll definitely find this topic completely polarized as it's very much peoples personal preference.
That being said, I'll now give you my completely bias opinion.
I have just recently switched back to FSX from X-Plane due to a single aircraft. I personally think the A2A C172 trainer is a game changer when it comes to simming, so I decided to switch back.
That being said, FSX is not my favorite environment to deal with. I find you have to spend a LOT more time tweaking FSX to get it to work properly, and you end up dealing with Add-Ons that interact negatively with each other and make a total mess of things. X-Planes add-on's API seems to be *much* more stable and you deal with conflicts a LOT less than FSX, it also has solid core functionality where you aren't having to use add-ons for features that make up for the short comings of a software package that is essentially end of life. (aka. FSUIPC, EZDOCK)
Also X-Planes add-on community seems a little less about making money and more about just being a solid community and providing cool stuff for people. I find that almost every "cool" plugin I come across for FSX, they're going to want $20-$40 for it. Most of the plugins I have for X-Plane were just free from community members.
I find with X-Plane, I could set it up for someone in about 30 minutes and they will be flying. With FSX, I started tweaking it to get it working "correctly" again on thursday of last week and I'm -STILL- tweaking it.
Again, this is my very very bias opinion of my experiences with both packages. To be perfectly honest, if A2A decided to switch all their planes to X-Plane tomorrow and ditch FSX I'd move back over in a heartbeat.