Ratio Of Magic Pen To Dmg League Of Legends

PenMagicLeague

So I'd like to know once and for all which is better - Flat Magic Penetration or Ability power?
I'd heard that Magic Penetration beats AP stat-for-stat. If this is true, why do I see so many people with Ability Power glyphs/Quints when the Magic Penetration ones would be better?
I understand Ability power will increase other things aside from the amount of damage a spell does, such as the amount of damage a shield will absorb etc. But is it really enough to make a difference?
For instance, I just got out of a 5v5 match vs an AP Sion, and the guy was a beast. He had a Doran's ring and was at around 39 Ability power at level 1. I checked his rune page after the game (I lost) and he has AP Glyphs, 2 AP quints, and 1 magic pen quint. Now, I understand AP will increase the damage his shield will absorb, but is 29 extra damage absorbed/dealt really better than 3.8 Magic pen?
If someone could post why they prefer AP over Magic pen for runes I'd really like to hear it. Because thus far I'm kinda stumped.

Ratio Of Magic Pen To Dmg League Of Legends Download

Mar 15, 2011  Thus dmg received equals (1-MR/(MR+100)) multiplied by the amount of damage of the spell cast. Assuming the MRPen equals 3.1.96 (ie. The amount received from 3 quints) Firstly, flat AP quints provide 12 bonus damage, assuming a.8 ratio, thus, you'd deal 12.(1-30/(30+100)) = 9.2. Anything that effects magic resist by a flat number can be used to reduce someone's MR under the 0 threshold. You need to have more flat magic penetration then they have magic resist. Sorcs and HG is +20 each Abby is effectively another +20.