First, just in case you weren't sure: finishing Act 2 with a character unlocks their faction-appropriate base-class buff for all your characters, including characters of the opposite faction. Any Republic characters you make will get the Knight buff in addition to their regular buff, and if you decide to level a Consular (for example), your Juggernaut will get the Inquisitor buff once your Consular finishes Act 2.
Either the Inquisitor/Consular buff or the Trooper/BH buff.
The Inquisitor/Consular buff gives +5% to primary stats (Str/Will/Aim/Cun) and, more importantly +10% internal & elemental damage reduction. The boost to primary stats means you'll do a little bit more damage and therefore generate a little bit more threat. Internal & elemental damage aren't nearly as common as physical damage, but 10% damage resistance can really make a difference in a long fight with a boss who inflicts those damage types regularly.
The Trooper/BH buff gives +5% Endurance. More endurance means more HP. More HP means more "cushion" when tanking; taking a crit, the healer messing up their rotation, you messing up your cooldown rotation, etc. are all less risky because you can take more hits before going down.
It doesn't really matter, though.
If you're in a flashpoint, odds are the 4 people will have all 4 buffs covered between them. If you're in a raid, it's a sure thing. Picking up additional buffs is more of an aid when soloing; that means it's actually more beneficial if you're planning on leveling multiple characters than it is for people who intend to concentrate on a single max-level character.