FSX was the first of the 2 mentioned flightsims.
Later "FSX" became "ESP" and Microsoft stopped developing ESP and the ESP-SDK:
- Microsoft ESP (FSX) SDK :
https://msdn.microsoft.com/en-us/library/cc526948.aspx
This statement appears to be quoted and claims that FSX became ESP and Microsoft stopped developing ESP. However the quoting link has no information about sources of development and no mention of FSX.
On 9 July 2014, Dovetail Games announced that it has signed a licensing agreement with Microsoft to release
Microsoft Flight Simulator X: Gold Edition on Steam in late 2014, titled Microsoft Flight Simulator X: Steam Edition. It includes content that was provided with the original FSX: Gold Edition which includes FSX: Deluxe Edition, the Acceleration expansion pack and both official Service Packs and repackages them in one bundle and a single installation.
https://en.wikipedia.org/wiki/Dovetail_Games
What is Microsoft Flight Simulator X: Steam Edition?
Microsoft Flight Simulator X: Steam Edition (“FSX: Steam Edition”) is the re-release of Microsoft Flight Simulator X Gold Edition (“FSX”) on the Steam digital platform.
https://fsxinsider.com/faqs/
Built on the core technology behind Flight Simulator X, Microsoft ESP is a visual simulation platform that brings immersive games-based technology to training and learning, decision support, and research and development modeling for government and commercial organizations.
Of specific interest to the Flight Simulator developer community is the opportunity to expand on the powerful Microsoft ESP serious games simulation platform which is
licensed to be sold by third parties and used in commercial, non-entertainment settings.
The best news of all for Flight Simulator add-on developers is that many of the same APIs are common between Flight Simulator and Microsoft ESP. Add-ons developed for Flight Simulator can be transitioned for use with Microsoft ESP with few issues.
https://www.microsoft.com/Products/Games/FSInsider/developers/Pages/
Dovetail purchased the rights to repackage FSX Gold with Acceleration and sell it on Steam. There is no mention of "FSnext." ESP is licensed to be used in commercial, non-entertainment settings. Software that works for FSX, usually also works for ESP. Both ESP and FSW were derived of FSX. The OP question was differences between development for the sims, not, "what is the history of the franchise."
To answer to OP question: The main difference is that P3D is a constantly evolving and changing platform. Something that works in FSX: Acceleration also works in FSX Gold. Something that works in P3D V4 may not work in P3D V1. So to develop for P3D, a person has to continually learn new technique, this is not the case developing for FSX.