Armor Penetration vs Damage(flat and scaling ad)

You got it wrong Wreckz because tanks have more armor, flat armor pen is less effective against them.

Because each point of armor provides less and less damage reduction the closer toe points you remove are to zero the more effective the armor pen is. This is also why magic pen builds go all out the more flat pen you have the more effective it is.

This means armor pen runes are strongest early when armor values are low. Flat AD on the other hand is really strong early to the point where it crushes armor pen early. Eventually armor pen catches up as you gain more ad.(since you won't be gaining flat pen typically while you will be getting quite a lot of ad.)

I haven't crunched the numbers since season 3 but the long and short of it was flat armor pen sucked hard. These numbers are from memory, but armor pen is worse than ad at level 1, dealing about ~10% less damage to champions and ~20% less to minions at about a hundred ad armor pen breaks even versus champions. On an endgame carry armor pen deals 5% more damage or so.

Certain champions fall well out side these parameters, but after the removal of old cleaver and the flat armor reduction (at the time reducing 70-80+ armor flat was reasonably doable. Nowadays breaking 40 is neigh impossible.

There certain situations where armor pen is better on specific champions. Mid lane assassins mostly, but by and large flat ad is more effective.