Familiar and mercenary. The content is balanced against the gear you get from the quartermaster quest, plus the mount, familiar, mercenary, and their respective infusions. The "free" mercenary (Hangar Voidstomper) is unlocked to hire anywhere and also level 20, so he's going to provide a better Mercenary Battalion buff than the level 1 and locked celestial mercenary.
If you have enough training reduction items, you can unlock the celestial mercenary to hire anywhere and also level it up to 20 and at that point, it'll be much better than Hangar. If not, then you're better off using Hangar while training the celestial merc separately.
For reference, if you gear up with the items from the quartermaster quest, using that mount, familiar, and mercenary, with the infusions from the lockbox, you should be at 41k-45k crit bonus and 1000-1200 fervor. If either of those stats are lower, then it's most likely because you're using an older familiar and/or an older (or not unlocked/leveled up) mercenary.
