ESF 1.2.3 is the most stable and active version.
ESF 1.3 Open Beta is the last public release of ESF before ESF 1.3 (ESF Final) is completed.
It is not rather stable or complete, thus it is not very active amongst the public.
It does not support bots. (it tells you this under the file description before you download it btw) The game crashes because the bots attempt to perform things which dont exist anymore, and they will not be updated.
I recommend getting into ESF 1.2.3 - it is the most playable and most played release, and should leave you adequately prepared for ESF Final.