In some ways MSFS is completely unrealistic. The flight models are probably fine on paper, but the aircraft do not fly as they do in real life. The feel is all wrong. The vision from the cockpit can be a limiting factor, and it can be quite difficult to get a sense of spatial orientation (for example when in the circuit) due to the fact that you can't really swing your head around as you can in real life. Something like TrackIR might be helpful there. The ATC I find to be an annoyance, rather than adding to the realism. I guess it's based on US procedures, and not really helpful to me, being based in Australia, with very different procedures.
After saying that though, I have used FS2002 and FS2004 for some of the procedural stuff, and it generally works fine (with limitations). In fact it has probably saved me quite a few bucks, as I have been able to practise a few things at home, rather than having silly mistakes or lack of adequate preparation cost me hundreds of dollars an hour to refly and get right. If it helps me learn a procedure through practise and repetition, without costing me a bucketload, I'm all for it.
Cross country flights can be quite useful too, although for VFR, you want to make sure you have some decent scenery downloaded or purchased for your region. Preferably with reporting points and local landmarks clearly visible.
From what I understand about X-Plane (without actually having tried it), the flight model is more realistic than in MSFS. Supposedly the flight models are calculated based on airflow over the aircraft, rather than from plugging in a mathematical model that "emulates" an aircraft. This approach would seem to provide more fidelity to the aerodynamic properties of the aircraft model, and would allow a potentially truer simulation of lift, drag, aircraft handling at various speeds, stall characteristics (which MSFS handles pretty woefully), spins etc. Dunno. I'll have to give X-Plane a spin to see if it is any better than MSFS.
I'd say that if you want the bells and whistles, and all the pretty stuff, then go MSFS.
If you are after a more realistic simulator, try X-Plane.
If you want a much higher level of realism, try some of the professional simulator packages.