How do you decide which to use? I pretty much use ad for ADC and bruisers and pen for junglers.
AD runes are good on some high-scale early game champs
for example, like +20 Ad gives 50% more damage on renekton E
depends champ to champ really.
Armor pen is pretty good on anyone with %armor reduction, like Wukong, Xin, Jarvan etc.
Its also really good on Nocturne because of all his free AD.
Its good on Hecarim because most of his damage comes from Q.
I run pen if I think I'll be fighting champions a lot early, like on Kha'zix. If I just want to farm a lot, like on an ADC, I'll take AD runes to make last hitting easier.
I run a mix of both.
So on pantheon...?
AD is good early on, even in duels, since overall it nets you more damage. Armor penetration is good later on, since AD isn't as hard to build as penetration can be.
So basically AD is better for snowballing, Armor pen is better for staying relevant later on. (Or if you don't have much scaling, so you're more of an ad tank)
I'd start AD on Pantheon since his e has such great scaling, and will help you get some early kills to get you rolling. As a contrast, I'd start Armor pen on Nasus.
I'm not a pro so feel free to take my advice with a grain of salt.
why not both? :^)
BEST ADVICE I EVER GOT:
If you have trouble last hitting, GO AD!
If you have no problems last hitting, and are playing adc's like Vayne, go ARMPen!
For me my last hitting improved DRAMATICALLY with AD runes.
Best IMO for every one is this - 2Arpen 7 AD.
Why? Because 2 arpen reds gives 2.6 arpen, but in game pen values round up or down unlike all other stats, so f0r 2 reds you get 3 armor pen which is hands down > 1.9 dmg
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