I'm not sure how one actually calculates Magic Penetration.
So.. For example: 
gives you 15 AP and 
gives you 15 Magic Pen
Does it mean that going two (30 AP with less money) is much better than going (without considering the movement speed)?
Magic Pen = AP? I need advice :)
Magic Resist is calculated like this:
If you have 50 Magic Resist you will receive 33% less damage (50/(100+50)=.33 If you have 100 Magic Resist, you will receive 50% less damage (100/(100+100)=.50 If you have 200 Magic Resist, you will receive 67% less damage (200/(100+200)=.67
So the formula is MagicResist / (100 + MagicResist) = Damage reduction
The way Magic Pen is taken into this is by taking the Magic Resist and subtracting the Magic Pen.
For example, if your opponent has 100 Magic Resist, and you have Sorc Shoes (+15 Magic Pen), they will only be taking 46% damage reduction from you instead of the 50% that it would be without the Sorc Shoes
Their Magic Resist is 100 minus 15 (for the Magic Pen from Sorc Shes) = 85.
Then you throw 85 into the calculator.
85/(100+85)=46% damage reduction.
Make sense?
Okay so the simplified rule is that high ratios to base power, you generally want to build a lot of AP. If you have high base powers to your ratios, you generally want magic pen.
[deleted]
Thank you guys all for the helpful informations! Now I know better how to manage AP and Magic Pen :)
I agree with TehNACHO:
If your damage abilities have low base damages but high ratios (e.g. 100 (+60/70% or more AP), stacking raw AP is more efficient.
If your abilities have high base damages but LOW ratios (e.g. 500 (+40/50% or less AP), then building AP is less efficient, since less of it will translate into ability damage. Magic penetration bypasses flat MR, and thus increases the damage you do with your abilities.
It depends entirely on your AP ratios. Stacking Dorans is silly though.
