Hi Dirk,
First of all, sorry to hear about your FSX misfortunes.
There is indeed a limited number of registrations for FSX (3, if I recall well). As far as I know, this should apply to all FSX versions. However, I am not quite sure how this works, it's possible that multiple registrations on the same OS and hardware (in Microsoft's acception of the "same hardware" term) count as 1 registration.
Nevertheless, it's rather bad for MS to do this with the last product in the line (thus "condemned" to be here for many years, and reinstalled again and again after hardware upgrades).
Actually, this was one reason for me NOT to perform my 3rd installation and registration. The other reason was that after using FS9 and trying to use FSX on the same hardware+software setup, FSX needed significantly more resources (even with most graphic sliders to the left end), and this may be the difference between usable and unusable. The third reason is that I am not particularly delighted to see cars on the roads or birds in the sky (that could come from my technical - rather than artistic - background)
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So, even today, I am still using FS9 for IFR flights and trying to use X-Plane for VFR flights. Why "trying"? First because in X-Plane it's significantly more difficult fo fly a plane (it's probably more realistic too, but difficult is difficult...). Then, there are some significant differences in the "look-and-feel" of the program, which make difficult to get accustomed to BOTH MSFS and X-Plane. For instance, it happened to me (twice!), during a VFR group flight, to click the wrong button when closing the map, and realize that I actually closed X-Plane (no warning/confirmation message). Last but not least, X-Plane crashes do happen; not very often, but more often than in FS9 (which remains the most stable of these products, judging by the crash count over many years).
However, I never tried an IFR flight with X-Plane, or any payware aircraft, to see what extra these bring to the flying experience. Yo need input on this part from more seasoned X-Plane flyers than me.
Basically, for online IFR flights I prefer FS9 for it's stability, and for (mostly offline) VFR flights I prefer X-Plane for its far superior terrain rendering (it's not only a matter of resolution and textures, but in MSFS I was driven crazy by the rivers that often flow uphill)
Andrei