Versions 2.XX are based on .NET Framework and can only run on Windows (7/8/8.1/10/11).
Versions 3.XX are implemented on the basis of .NET Core + EtoForms and were planned as a complete replacement (cross-platform) for Windows, Linux, MacOS, etc. - with the complete cessation of development of 2.XXVersions 3.XX are implemented on the basis of .NET Core + EtoForms and were planned as a complete replacement (cross-platform) for Windows, Linux, MacOS, etc. - with the complete cessation of development of 2.XX.
But over time, it was discovered that .NET Core is constantly removing support for decommissioned OS versions, including Windows. Because of this, there are numerous compatibility issues between GEDKeeper 3.XX and Win7/8/8.1.
Therefore, until the moment when (or if) there is nothing to choose from (restrictions, loss of the ability to use some option) - it was decided to develop both lines. This is very labor-intensive. But if in Linux you can only use 3.XX, then in Windows it is inconvenient to use it - and this is on Win10/11.
So for all versions of Windows I recommend using 2.XX - it is more stable and the interface is more efficient.
All the differences concern only the interface. The core is completely identical, i.e. all the functionality is identical, not related to the appearance and interaction with the user. And both lines of versions are compiled simultaneously from the same code base during releases.