X-Plane is great, but it really doesn't have the support like FS9 or FSX for addon professional software. PMDG,Level-D,Captain Sim, Wilco Publishing etc don't make aircraft for X-Plane so it's a no brainer for IFR. Then there is the better world coverage by FS9/FSX landscape,terrain,airports etc.
Personally if your into plane design then X-Plane for sure fantastic product.
VFR - FSX graphics are fantastic! (great for IFR as well since the latest service packs)
IFR - FS9 (currently better support for Vatsim software)
Vatsim for added realism, once you start flying online with real ATC flying offline feels very boring and only good for practicing circuits or getting use to a new plane. Plus MS ATC in FS is terrible!! Radar Contact isn't much better. There is no substitute for real ATC sometimes I even get more jitters flying online via Vatsim then I do real world.
Bascially for the best experience imho, get some practice flying twin props, go online with Vatsim get some hours up,join a Virtual Airline and start a career. Won't be long and you'll be flying 744's in and out of major airports. And hand flying SID/STAR's.
Then again maybe all you want to do is Bush Flying in Alaska.

Have Fun!
<S>...-Gixer