Since lethality is flat, and armor pen is percentage, ima assume lethality vs squishies (which is who assasins go after so they build it) and armor pen vs tankier champions, (who is the job of the adc to deal with, so they buy pen). Is this correct?
waiting for WoW2 votesPercentage armor pen is good on squishy targets as well since their armor increases enough with levels, and a lot of squishy champs buy things like Ninja Tabi.
Lethality is overall the better early to mid game stat but doesn't feel as strong the longer the game goes on, when a lot of carries get things like tabi and guardian angel. You'll have an easier time laning and a better mid game with lethality.
iPooUnicorns1 votesIf enemy has no armor: 
If enemy has lots of armor: 
If it's an enemy: 
Side note, 
still functions like the old last whisper, as in it's always a good buy.
Narsicus1 votesThat is the basic take away, yeah.
The more armor a person gets the less value flat pen (lethality) gets, and the less armor a person has the less value percentage pen gets (especially last whisper which only affects bonus armor).
So if you are up against a team of enemies who you think are unlikely to be investing in armor an early lethality build can be very strong, but if the enemy has champions that will be stacking resistances you might want to switch to a different build because lethality will fall off as they stack up armor.
Bronze Demon1 votesarmor pen is good vs those with high armor rating BUT most or a good portion of that armor must be bonus armor
lethality is good against anyone with low armor rating regardless of base/bonus armor
armor shred is good against anyone with medium-high armor rating regardless of base/bonus armor
it is often good to combine two of these three types of pen either using either runes, items, or champion abilities.
