The official finished version is MoM 1.42.
I developed Xtended Final as a modmod for MoM 1.42.
There is a new version: MoM Alpha, which has many new features and it is official from MoM Team. But, it is still in alpha stage (various features are still in development/not implemented yet, still need various tweaks/balances etc). Sephi and team are developing next version of alpha right now.
I developed Xtended for Alpha (the one in this thread) as a modmod for the MoM Alpha. It contains tweaks, balances, additional contents etc. It is basically my interpretation on "how a finished MoM should be" (within the limits of MoM Alpha's mechanics).
There might be some contents from Xtended for Alpha that will be merged in the next version of official MoM (currently being developed), but we have not discussed which one(s).
Which one you should play?
Well, it depends. If you like to play a "final version", play 1.42 (with Xtended Final if you want more religious contents and civilizations unique flavor).
If you want to play MoM Alpha, go ahead! It is stable with no major bugs that I know of, playable and introduces many fun features not in 1.42 such as Guilds. You can add Xtended for Alpha for more polished features.