The realism in X-Plane (9) isn't really up to much compared to FSX (neither have a convincing fully developed stall of a Cessan for example), and out of the box, XP9 is un-flyable (turn the turbulence down/off as it's bonkers to start with), and looks terrible (turn the quality settings up immeadiately), and has the lowest amount of quality control imaginable, some of the aircraft interiors are just shockingly bad (and half the cockpit controls don't work/pointless GPS etc). What XPlane does have, is non-flat runways which are quite nice. And look for Carenado add-on aircraft as the default are so poor.
XP10, just about released (3rd isn't it?), isn't any better than FSX imho, for a variety of reasons which you won't be worried about as they're to do with the environment rather than the simulation. The Demo puts you in a 747 with the viewpoint so far forward you can't zoom back to see the panel, and performance isn't nearly as good as XP9.
If you are on Windows, just get FSX - it has the most massive support base and add-ons imaginable, and is far far easier to get setup and working than XPlane for a casual user. And it has a useful GPS for crosschecking where you are when necessary, and the weather generation is ok, but ActiveSky is the add-on if you want proper live metars as opposed to an FSX approximation.
As has already been said, Vatsim is where it's at for proper ATC, or RadarContact 4 which is a replacement for the built in FS one really. And a TrackIR is insanely useful.
And as someone's already mentioned FSPassengers, might I also suggest Air Hauler as a worthy addition if you want something that will tell you where to go (under the guise of running a cargo company) rather than fly around willy nilly.
(for either FSX, or XP, but I'd recommend FSX).