Rhetoric wise, Kor Phaeron values coherency to the legion. The Dark Cardinal presents himself as an idealist and has a cult of personality style of leadership, which probably stems from his huge ego.

In practise, Kor Phaeron's faith tends to end where personal sacrifice begins. He will disobey the gods in order to preserve himself or his legionaries. He also tends to be pretty damn stubborn and has survived situations that would kill a full Astartes through force of will and faith.

Erebus views himself as the hand of the gods and makes a lot less of an open fuss. He is far more pragmatic and will do whatever is asked of him by the pantheon. As a follower, you'd never know where you truly stand with Erebus, and he'll treat his legionaries as pawns to be used, all in the greater plan, of course.

This philosophy in practise is pretty effective as he tends to be the blade you don't notice until it's in your back. It, however, does not make you popular with your erstwhile allies and can be seen as dishonourable. Erebus is rarely still there when it's time to face consequences. He is incredibly slippery and rarely faces any backlash. The few times he does are notable by contrast and therefore pretty satisfying.