Is one of them more viable for Late game? Early game? Last hitting? Harassing enemy?
AD runes allow you to last hit easier, and does more damage earlier on, and works better if you have AD scaling abilities that do magic damage. Armour pen allows you to harass the enemy harder, and scales better into late game.
Late game, Armor pen all the way. Early game for last hitting. AD.
Chaox likes to run AD quints and reds, and then his yellows and blues are customizeable. The ad helps with last hitting, which helps with late game. :)
Chaox will run AD quints and reds on everyone but MF in which case he runes apen reds. The bonus ad scales with most abilities and lets you last hit better.
It depends on your play style, do you have a hard time last hitting or want to make sure you get every single one then AD is for you. If you are really aggressive you also want to get AD runes because it benefits you the most against other low armor squishes. Usually by the time that you need armor penetration you will be building a last whisper. The thing about armor pen runes is the are not as effective if you take LW because that is taken before the % is taken off. Make those runes not take off their entire amount.
I go Pure AD on most champions because I am aggressive and need all the extra damage on my abilities when I kamikaze in and heal bait.
There are many analysis on the internet I've read. It depends on
- Items/your build style
- Enemy build + champions
- If you want an early or late game advantage
- AD scaling for your champion
- Your build style
- Your lane partner and opponent
- Your team (solo or pre-made)
My personal choice (I like playing AD carries) is AD (Caitlyn, Graves), perhaps with AS (Twitch, Graves) or Crit Chance (any)
Some random thoughts...
- I build Crit Chance and AS before AD: a few early game crit hits can force your lane opponent back and they lose gold/XP.
- Caitlyn's E scales with 1.3 AD, her R with 2.0 bonus AD. So AD is good for Caitlyn.
This applies to other champions so this makes AD more effective for longer
- AS + Crit works for, say, Twitch to get the stacks for Expunge
- Early game last hitting and some kills gives more gold to replace the ArP by items
- ArP runes can reduce the effectiveness of, say, a Last Whisper
- I can buy ArP as needed (eg Brutalizer, GhostBlade)
- I rarely buy Last Whisper because too many folk go DPS and don't bother with survivability. I do it too
- When the AP and AD carries are all killed, then your team is free to kill the enemy tank who survived. The AD and AP carries can't stack armour because then they lose DPS.