Flat AD/AP over Penetration AD/AP during the laning phase
A very famous French streamer, and former LoL pro (EC Skyyart) always says in its streams that going for penetration items (AD/AP depending on your champ obviously) when your opponent has low armor/MR is always more worth it than going flat AD/AP. At least in early game (during the laning phase) in order to deal damage close to the “true damage“.
I wonder if that is really the case when computing some calculation and reflecting over the theory. Please keep in mind that I speak about the laning phase and don’t include the runes in the question.
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This depends on your champion and Playstyle. While for most AP based characters his statement is true, it doesn’t apply to all AD characters.
The main reason why Penetration stats are strong against Low-Resistance opponents is (as you have already mentioned in your question) because of the “True Damage”. Obviously you won’t really deal true damage to an enemy (or at least not often) but Armor or Magic penetration will increase your Damage output significantly if you are playing a spell based champion.
Most spell based champions do scale heavily with spell-level rather than with AP/AD. Obviously there are some exceptions to it but as long as you don’t have a 0.8 scaling + on at least 2 spells spell levels are stronger than flat AP/AD.
Because of that buying flat penetration like found on Brutalizer or Haunting Guise is exceptionally effective for harrassment during early and even mid-game. It allows you to snowball and while flat Penetration isn’t uber strong late-game it’s still decent and certainly not wasted.
The biggest disadvantage to it is that lasthitting might get a lot harder since flat AD/AP is the stat to go with there. However if you are comfortable with your lasthitting skills you can safely go for Armor/Magic Penetration early for more aggression in lane.
Please Note that this is really only an advantage on Casters. Autoattack (Ashe/Jinx) or extremely high scaling characters (Zilean/Anivia) are usually stronger with more of their Flat damage stat.
I guess it depends on how you want to go with your early game.
If it were me, I’d get Flat AD/AP for last hitting minions. since I particularly want to earn up on gold and exp in the first few waves.
With Flat AD/AP Penetration however, you’ll get a bit more of an edge when dueling / trading with your lane opponent. (especially if your opponent has low armor/MR just like you mentioned)