I bought X-Plane v8.21 and I have to say, it's pretty damn impressive. The visual models of the aircraft still need catching up to do, but the dynamics and scenery really blew me away.
One of the major things that I like about it is the way is manages the graphics. Whenever the sim slows down [due to limited resources], the sim will automatically reduce the graphics by simply creating enough haze to help gain back some of the needed resources. It even comes with texture compression and can remember the background scenery [the scenery beyond your field of vision] without having to reduce and reload it like FS does.
Downside: Even with DVDs, the sim still requires multiple discs for the entire world.