X-Plane because:
OpenGL based, runs on Mac, Windows and Linux, very active community, smooth instruments, payware is cheap and very good, flight dynamics are superb, extremely good world wide scenery, network functionality embedded, easy to use SDK for development of own plugins, nice scenery development tools, comes with an aircraft builder, easy to repaint aircraft, compatible to G1000 (connect and go), used by professionals as well as home users, more fun for less money.
And just that you see that I know what I´m talking about:
I fly real world microlight aircraft, I ran an MS FS 2004 Setup on three (!) computers (one for the sim, one for all the needed tools like ActiveWather, IVAO and Vatsim Communication Tools, FSKeeper, and so on, and one for the charts. I spent about 1000EUR for the Sim and all the addon software like the PMDG 737 and 747, Airports all over the world, and so on. I wouldn´t say that it´s bad, but one Mac is enough to run X-Plane with all the functions I need and i can be sure that the computer does not crash when I rach my destination after 12 hours of flight. When I saw the first FSX videos I was looking forward to it, when I saw the demo I canceled the pre-order. FSX is nothing else than a slightly overhauled version of FS 9. For me this was the end of MSFS after flying in XP and MSFS the same amount of time. XPlane needs less resources for better results (180fps vs. 30fps) and runs on every computers in the house.
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