Armor Penetration vs Damage vs Combat Advantage damage

In practice, what combination can achieve the most damage?
What is the real difference?
Ex.: Armor Penetration, Ignore % of Armor?

Example Warlock Skills:
- Skill 1 (Passive): +20% Armor Penetration (Prince of Hell)
- Skill 2 (Passive): +12% Damage (Flames of Empowerment)
- Skill 3 (Passive): Combat Advantage damage (No Pit, No Mercy)

In practice, what combination can atirngir the most damage?

From what we tested:

• DOT: % Armor Penetration is good.
• Direct damage: % Damage is good.