This is an interesting thread, and one that I have been trying to keep up with.
Firstly, a general note.... If you are looking for a game that is 'complete' and has all official rules produced by GW, then none of the Specialist Games tick that box I am afraid. In general, all of the games were abandoned before they were complete and the incredible SG community have picked up where GW left off in all cases. Now, part of this is because of the way that SG was originally set up. When created, the development of the games was a collaborative process from the very beginning. All SG games had a rules committee consisting of Jervis and a number of key players of the game. Each year, the committee for each game would 'meet' (virtually) and agree on core rules changes and development direction. When GW fell back from SG and their development, these development groups/committees simply continued. For example, BFG has the High Admiralty and EA has that ERC (Epic Rules Committee), and although the membership of these groups may have changed (I think that the HA are the original members but that only nealhunt is retained from the ERC) their mission is unchanged.
With Epic, aside from the ERC, each force had an Army Champion. With so many so work on, official development was limited to Marines, IG, Orks and Eldar, with Chaos added later. Other ACs were appointed by Jervis personally and development happened at TacCmd, with the view that development could be moved to the SG boards when a miniature line was developed and GW were ready to produce the force, and it would be mostly done by that point as well. Unfortunately, the reverse happened, GW pulled back and the official development in the other direction, onto TacCmd, and then the SG boards were also moved to the same location.
I guess that what I am trying to say is that all SG games have a percentage of fan-development. And, this fan development is taken seriously and is not the same kind of fan development that you will usually see in 40K (apologies to any 40K players that I may offend with this). As a community, we are aware that developing serious, balanced and professional lists and rules is something that will make or break the games that we love. I understand the reluctance to accept fan development, and the reasons for this, but it is worth giving it a go with the SG games.
Also, I served as the AC for the Tau for a brief period, so I can hopefully help there. In general, the progression of rules and lists is that the 'most official' are the lists produced in the GW suppliments. The NetEA/TacCmd lists are in a state of development following these examples. The EpicUK lists are developed specifically for the tournament scene in the UK, in a response to the perception that some official or NetEA lists worked better for tournaments with a few minimal alterations. I am sure that EpicUK will produce all lists eventually, and currently the EpicUK Tau writer is also the NetEA Tau AC, so I am hopeful that a single list will be produced for both sets of players.
Development of the Tau has been ongoing since the beginning (I still have the email from Jervis somewhere about the Tau AC). Around version 3.4 of the Tau list, FW produced the IA3 book. This contained a version of the EA Tau list which was extremely similar to version 3.4 of the NetEA Tau list, but with a few alterations in the rules (for example the Manta) and formations changed to meet the current FW Tau pack sizes. All speculation on the origin of the IA3 list will end there, except to say that the NetEA Tau list is now on version 6.4. The IA3 list is generally considered unbalanced in a number of areas. (One example where fan-development is considered an advantage over official, since we can continue to adjust and test.)
For both EA and BFG, the rules are stable. Even GW produces alterations to their core rules gradually, and that process is likely to continue with both EA and BFG, but the games are balanced in their core rules and accepted generally. You can go to someones games club and play the same rules of BFG with only a quick discussion on whether you use MMS, and the new attack craft, reloading and Nova rules. Epic is almost as easy, but a little more complex as the organisation of the rules has meant that there are a few more differences. There are the core GW rules, a set of updates, and a revised rules set, known as the handbook.
Anyway, I am pretty sure that no matter which of the games you decide to pick up, you will be back to pick up the other one shortly after!
