In lots of forums, ArPen runes are considered the best marks (along with Insight) and AS runes are referred as “do not buy runes”. I’m not quite sure why.
9 Tier 3 ArPen runes comes to 11.61 ArPen. Surprisingly, the more Armor an enemy has, the less effect these runes have. I did the maths:
Let’s say you have 100 Attack Damage.
If your target has 25 armor (20% reduction), you will deliver 80 damage without ArPen runes.
With ArPen reduction of 11.61 you decrease an enemy’s armor to 13.39 (11.8% reduction) and you deliver 88.2 damage. So your ArPen runes gave you 10.2% bonus dmg (88.2 / 80)
But if your target has, let’s say 75 armor (42.9% reduction), you will deliver 57.1 damage without runes.
With ArPen runes, your target has 63.39 Armor (38.8% reduction) and you will deliver 61.2 damage, which is only 7.1% more than damage without runes (61.2 / 57.1).
150 armor (60% reduction) = 40 damage //
with runes 138.39 armor (58.1% reduction) = 41.9 damage //
difference = 4.8%
So we can say, ArPen runes will give you +5-10% extra damage depending on how much armor your enemy has. But AS runes will give you 11.88% more attack speed.
ArPen will further benefit from attack speed and damage. But it only stacks with more ArPen. AS will further benefit from damage and ArPen, but will only stack with more AS. And you will also benefit more from hit-to-apply-affect skills. Of course, AS will not help you much for champions with strong physical skills, like Riven or Garen.
It seems to me that ArPen is better for champions who use lots of AS items or have strong physical skills, but it will be a lot weaker for champions who use damage or ArPen items.
Did I miss something? Because if you find yourself battling against some heavy tank, like Rammus, you can end up having all your marks for ArPen giving you only some minor 3% bonus, instead of a nice 12% attack speed bonus.
First, let’s clear up a misconception.
Armor Penetration gives you a constant %damage bonus, regardless of how much armor the enemy has see here.
It doesn’t matter if your enemy has 300 armor or 15 — penetrating 10 of that will have the same effect on total DPS increase.
That said, let’s take a look at why Armor Penetration is preferred, even with its linearity.
Attack Speed is easy to acquire via items.
Armor Penetration is not.
You can more than match the effects of attack speed runes with a single dagger – 420 gold.
The cheapest item with Armor Penetration is the Brutalizer at 1337 total gold. By this metric alone, Armor penetration runes are worth approximately three times that of attack speed runes.
It is easier to use items to stack Attack Speed over Armor Penetration — but to get the highest DPS, you want to stack both.
Given that the amount of Armor Penetration available via items is strictly limited, if one’s goal is to achieve maximal values for both Armor Penetration and Attack Speed, runes for Armor Penetration are much better than that of Attack Speed.
Source : Link , Question Author : Slayer , Answer Author : Community