I fly both as well, at least since I picked up XP in June.
To be honest however, since picking it up, my hours in XP have outnumbered my hours in FSX by ten to one.
It helps that the area I like to fly in (Western Canada) has exquisite mesh, very good to excellent OSM data coverage (I can not only spot my house, I can spot my detached double car garage!!), and very near complete (and fairly decent) small and large airport coverage available via Tom Curtis's packages.
Mountain flying, in my opinion, far surpasses FSX due to the crisp high resolution textures everywhere and into the distance. Last night, for example, I flew the Alabeo 195 on a FSEconomy flight from CYVR to CYLW, taking off at 6 am. It was pure heaven. As a plus, both my departure and arrival airports were quite fleshed out, with proper buildings and pretty good photo-real bases.
In fact, although I used to be quite regular in the FSX forum, I haven't been posting there so much because I just haven't been flying that sim as much. Pretty much the only time I do so is to fly certain aircraft on FSEconomy flights that either aren't available in XP, or that I simply already own in FSX and have no desire to buy again.