Pantheon Flat AD vs Armor Penetration Runes

Hey guys this is my first post so please be easy.

I've been playing Pantheon since he first came out and I main him top and mid since season 3. I go 21/2/7 for masteries. The problem is my runes. I've been using Flat AD runes so I get +22 damage at lv 1. It seems great and strong because you do tons of damage with your Q around 110, I think. But I've been wondering, is it better to switch out some Flat AD runes for Armor Penetration runes? And which ones? Because I know the less armor the enemy champion has, the more damage they receive depending on the armor scaling curve. So what's the best combination to reach maximum potential damage with both types of runes.

Thank you for your opinion and hope to see you all on Summoner's Rift.