Originally Posted by EasymodeX
Your analysis is fundamentally flawed because of 3 points:
1. In games where your spell damage matters and it is your primarily damage output, and you are successful enough that this discussion is relevant, opponents will have approximately 100 MR (Merc, Banshee, base MR).
2. Mag Pen augments the entire damage of your spell -- both the base damage, and the AP bonus damage. You do not appear to have accounted for the base damage in your analysis, which comprises the majority of the spell damage, marginalizing the benefits of AP to a certain extent.
3. AP bonuses in terms of alternative cost is additionally reduced due to most spells in various situations having a <1.0 AP ratio.
So, as an example,
Assumptions: Base spell damage of 250. AP ratio of 0.8. Target with an MR of ~80. Base AP of 100. 28.5 flat magic pen (Sorc boots + MPen Marks). 15% magic pen (mastery).
Base damage: 330 minus whatever 39.5 MR does.
+Void staff: 386 minus whatever 7.5 MR does.
+70 AP + 33.5 AP: 413 minus whatever 39.5 MR does.
I'm guessing that reducing the enemy MR from 40 to 8 will make a much larger difference than increasing your base spell damage from 386 to 413, tbh.
Thought I'd go ahead and plug in some numbers so this makes a little more sense.
By the way: Damage Reduction = Total Armor / (100 + Total Armor). For example, a champion with 150 points of armor would receive 60% reduced damage from physical attacks.
MR scales the same way armor does. So! Using your example:
80 MR would set reduction at 44% With your pen factored to bring MR down to 39.5, that makes it 28%. So 330 is mitigated to 238.
Void staff brings reduction from 28% to 6% so 330 is mitigated to 311.
Next example, 413 damage versus 28% reduction brings it down to 298.
So the magic pen does more damage, not a whole lot, but more. One of the things you have to consider when deciding to go with pen or AP is, naturally, your other skills...like if you had a heal, for example, I'd say go with a bit more AP since pen doesn't let you heal more. Those cases are usually somewhat rare, though, since a majority of your mages are just that; mages, and you don't have to worry about anything aside from pumping your damage.
So, whats better to lead off with during your lane phase? AP or Pen? We'll go with saying that the spell does 80 damage, seems to be a common level 1 nuke damage, and has a ratio of .7 (basically Kassadins nullsphere).
With full pen runes, you're getting 19 magic pen, plus 15% from the mastery versus 30 MR early on.
30 MR is a 23% damage reduction, taken down to 8% from runes/masteries. So your 80 damage nuke ends up doing 74.
Now we go with full AP page (flat), and the 15% mastery. Full page gives you 31 extra ability power, making your nuke go from 80 damage to 101, or 81 damage after mitigation.