So is Diaspora using 3.6.12 or 3.6.13 and what are the differences and would any of them affect TBP?
3.6.13 is the dev branch for the eventual 3.6.14 release. But this is complicated by the fact that current 3.6.13 builds - including Diaspora's build - are far more advanced than any of the 3.6.14 release candidates, meaning that if 3.6.14 came out right now, it would be
way behind the Diaspora build and 3.6.13 as a whole.
At the moment it looks like we may just skip 3.6.14.
The differences are extremely substantial in terms of graphics and performance, as well as the power available to FREDders and modders. I don't know how significant any of them would be on TBP though.
Even the Diaspora build is now technically outdated. Internally I'm sure most high octane projects (Diaspora, BP, BtA, WoD inclusive) are using newer 3.6.13 builds.