Just wondering what you guys might think of this rune page for ADCs:

3 Armor Pen Quints
9 Armor Pen Marks
9 Armor Seals
9 MR per Level Glyphs

Most people take early AD for their Quints and Marks. I'm not entirely sure why. I use all Armor Pen because I usually don't have an issue with last hitting and I play extremely safe early game.

These scale much better into mid-late game than flat AD runes correct?

Depends on when in the game is your ADC's strong point. For caitlyn, it's usually better to go flat AD. For vayne or Kog, I'd say go arpen.

Why choose? I have a page for both. Flat AD is great for characters like Graves, Ashe and Cait, but again, Kog and Vayne are all about the mid-late game, so ARpen are probably more useful for them.