So about those new %Pen Masteries...

Deciding between %Armor Pen and %Magic Pen really isn't a choice.

Like...

At all.

On the vast majority of champions, anyway.

How about combing them, making it %Bonus hybrid pen, and bumping up the value a little?

Then toss in a stacking flat resist shred effect for its companion. Two per hit stacking up to 8 or so?

Now you're deciding between Tank Buster and Squish Buster. Or stick the stacking effect over in Not-Utility and the flat bonus in Not-Offense. Whatever works.