Microsoft's Flight Simulator X just appeared out of nowhere with its own "Steam Edition" for $4.99USD (currently 80% off). The price is right for a game originally released in 2006, but in one of those funny situations with older IPs two companies currently have the rights to release the game, and one of them is defence contractor Lockheed Martin. So which one's better?
The short answer is, the Steam version. There doesn't seem to be much reason to go the other route, whether you're a gamer, hobbyist, or aspiring pilot. While Lockheed Martin has been very, very slowly plugging away at the code for Flight Simulator X, gradually improving it so they could sell professional flight simulator "solutions" to schools and military outlets, Dovetail Games seems to have done a lot more with the game in a much smaller amount of time. That probably comes from assigning more than a handful of people to the job.
Dovetail is currently looking after the entertainment licence, having secured it recently after Microsoft ditched the franchise years ago, while Lockheed Martin owns the commercial sim licence. Under the latter licence, Flight Simulator X isn't allowed to be considered a "game", which is why it's taken so long to bring the popular sim to the sandy shores of Steam. There are others out there, of course, but some will prefer good ol' FSX, and there are years of community support that go along with it.
The Steam community response has been very positive so far. Hopefully Dovetail's recent coding sprint is a sign of support to come.