All cabal leaders gain a bonus to cabal powers, enhanced vitaility (leader con/age), access to additional cabal channels, cabal maintenance powers, and drastically increased immortal attention.

Additional bonuses by cabal:

Battle:

Leader weapon, Poultice, Trophy, Discern.

Fortress:

Maran leaders get a leader weapon. (Essentially a nodrop lightforge with slightly better stats)
Acolyte leaders get bless water.

Heralds:
Find out in game.

Scion:
The increase in cabal power efficacy is especially noticeable.

Tribunal:
Bonus abilities are based on your specific position. These bonuses include additional cabal powers, a modified jurisdiction, and lieutenants.

Empire:
Additional cabal powers based on sect.

Nexus:
See Scion.

Outlander:
Leader weapon and insect swarm.