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.
AzerothCore, CMaNGOS, and TrinityCore have a great feature that retail hasn’t, a true PvP statistical system.
PvPstats is a tool written by ShinDarth, released under the GNU-GPL3 license, that allows to store and display data about Battleground scores.
It consists of two parts: a core modification and a web application.
Core-part of PvPstats