FSX uses custom scenery, which has better quality (not as geometry, but
better enviroments). Graphically, since FLightgear 3.0 you can use the new
Renbrandt renderer and is on par with FSX.
Of course you should activate Terrasync, increase a lot the default draw and
quality distance and have a LOT of RAM to load those graphics. 16gb and a
powerful Intel graphics card or a *very* well supported nouveau card with
current MESA and kernel drivers.
Seriously, Flightgear can be *very* taxing, as much as X-Plane and more.
http://www.flightgear.org/wp-content/uploads/2015/05/shuttle.jpg
The Shuttle, low orbit.
http://forum.flightgear.org/viewtopic.php?f=5&t=22623&start=15
Custom airport models
And look the image: those clouds are automatically generated from the
*actual* real world weather from METAR.
FG is just awesome, it has modules to connect even with ADB-S and GNURadio.
And maybe you can see your real time position in the latest Marble , but I
have to check the its KML support .