Youmuus or lord dominik`r regards

on what champ vs what champ and items? there are alot depending on which you would build.

Flat armor penetration is for reducing the armor of Champions who aren't buying any as close to 0 as possible. % Armor Penetration is for shredding through the armor of tanks once they've stacked a good amount of it up.

When you should build either is heavily dictated by which champion you are playing, and also what your lane opponent/the enemy team is building.