League of Legends (LoL) Question: AP vs Magic Pen for some champs?

To go with your Elise example, she does a metric ****ton of base damage with her kit. the % current/missing health damage on Neurotoxin/ Venomous Bite is going to benefit far more from magic pen then it will AP as its ratio is terrible. The same is true of most of her abilities (all except Volatile Spiderling).

This concept can be used for any champ. "Does my champ have good base damages or good scaling?" is the question that will determine whether Magic Pen or AP should be bought. Most supports and heavy utility mages want penetration whereas most burst mages will have great AP scaling and want to buy days of AP and a Void Staff. Hope this is helpful :)

People with bad AP ratios tend to like magic pen better. People with good AP ratios forego magic penetration a little bit