Branch main is the one where new features are added to once they are completed. Branch support/3.0 contains features backported from main as well as fixes to Zato 3.0.
For some time, there have been no changes that could not be backported so both of them were in sync. But now all the internal changes to SimpleIO and support for Python 3 will be added to branch main only whereas support/3.0 will continue to only receive fixes or other smaller changes for as long as Zato 3.0 is supported.
If you want to have all the newest features - use main. If you want to apply stability fixes and some features - use support/3.0,