The 6.x branch of TrinityCore has been created almost 11 months ago and according to the TrinityCore forum download counter, the 3.3.5 is still the most used branch by the TrinityCore community.
In fact, all of the download counters (week, month and year) are showing that the 3.3.5 database has (the more or less) twice of the downloads of the 6.x one.
Right after, there are the old 4.3.4 database and the tools Keira2 DB Editor, SAI-Editor and TrinityCore JSON API.
Be aware that all files listed in those download counters can be downloaded via GitHub. For example, users can download the database from this page, but the counters showed above can just keep trace of the downloads performed via the TrinityCore forum.
Even the 6.x is not yet fully playable, its development is very active:
… and Shauren, one of the TrinityCore lead developers, is pushing more than 1 commit every day since April:
Good job to the TrinityCore team and all contributors, keep up the good work!