How is Armor pen not good late game? If you don't have armor pen you don't do as much to champs with armor...... You get a static 25 arp with reds and quints with a brutalizer you would counter a chain mail. With Vayne that is extremely important. She's an assassin and can burst champs with out defense pretty hard. That's my opinion and my play style.
I got the two mixed up. Armor pen is better late game, while Attack damage is superior early game.
From LoL forums:
Armor pen or Attack damage?
Current greater mark of strength gives
+0.95 damage X 9 = 8.55 damage
Current greater mark of desolation gives
+1.66 armor penetration X 9 = 14.94 armor pen
Time to whip out my calculator
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Because I have no idea how to make a spreadsheet, I shall be assuming several different scenarios.
1. Tristana has 50 damage (roughly level 1)
2. Tristana has 100 damage (roughly level 9 with basic items)
3. Tristana has 300 damage (roughly level 18 with farmed up stuff)
(keep in mind that crits alter the damage of both armor pen and direct damage equally, so I will not put these into account)
Tristana will be attacking
1. Fiddlesticks with 30 armor
2. Garen with 100 armor
3. Rammus with 300 armor
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Scenario 1
Tristana 1 (50 base damage) with +8.55 damage
VS fiddlesticks
58.55 X (100/130) = 45.0 damage
Vs garen
58.55 X (100/200) = 29.3 damage
Vs Rammus
58.55 X (100/400) = 14.6 damage
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Tristana 1 with +14.94 armor pen (I'll assume 15 armor penetration for the sake of not giving me a headache)
VS fiddlesticks
50 X (100/115) = 43.5 damage
Vs garen
50 X (100/185) = 27.0 damage
Vs Rammus
50 X (100/385) = 13.0 damage
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So in scenario one it goes...
Enemy-----Damage-----------Armor pen
Fiddles-------45.0-----------------43.5
Garen---------29.3-----------------27.0
Rammus-----14.6-----------------13.0
tl;dr raw damage is better early game
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Scenario 2
Tristana 2 (100 base damage) with +8.55 damage
VS fiddlesticks
108.55 X (100/130) = 83.5 damage
Vs garen
108.55 X (100/200) = 54.3 damage
Vs Rammus
108.55 X (100/400) = 27.1 damage
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Tristana 2 with +14.94 armor pen (once again simplified a bit)
VS fiddlesticks
100 X (100/115) = 87.0 damage
Vs garen
100 X (100/185) = 54.1 damage
Vs Rammus
100 X (100/385) = 26.0 damage
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So in scenario two it goes...
Enemy-----Damage-----------Armor pen
Fiddles-------83.5-----------------87.0
Garen---------54.3-----------------54.1
Rammus-----27.1-----------------26.0
tl;dr Armor pen and raw damage equal out around this point when you have ~100 damage while the enemy has ~100 armor. Raw damage is still a tad bit better imo.
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Scenario 3
Tristana 3 (300 base damage with big guns loaded) with +8.55 damage
VS fiddlesticks
308.55 X (100/130) = 237.3 damage
Vs garen
308.55 X (100/200) = 154.3 damage
Vs Rammus
308.55 X (100/400) = 77.1 damage
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Tristana 3 with +14.94 armor pen (once again simplified a bit)
VS fiddlesticks
300 X (100/115) = 260.9 damage
Vs garen
300 X (100/185) = 162.2 damage
Vs Rammus
300 X (100/385) = 77.9 damage
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So in scenario two it goes...
Enemy-----Damage-----------Armor pen
Fiddles-------237.3---------------260.9
Garen---------154.3---------------162.2
Rammus-----77.1-----------------77.9