For me what matters is that MaNGOS is here since 2005, and the project is constantly growing with a mature, friendly, and reliable community. Add our branches for WoW 1.12, 2.4.3, and 3.3.5a plus our development branch with stuff to try out 4.x (yes, we have a client patcher on github).
All of these are delivered to you with love and dedication.
I do not care for Trinity, Cactus, Skyfire, whatever. Those projects are all different, and have different goals, and very, very weird communities in some cases, too.
MaNGOS is here to give you the opportunity to grow your skills and knowledge. Trinity and others are not here for this reason. WCell may be the only exception where it's all about the experience and not just gaming for free.
The people doing development are all here on the MaNGOS forums. The team is the same for the last few years with a few additions, and no losses.
What people do not see and probably do not understand is a different thing: yes, there are forks and different projects, but these are connected these days. There are Trinity developers active on the MaNGOS forums and vice versa.
In general, if your goals lay somewhere around gaining knowledge, you're at the right place here, else it is not MaNGOS you want.