Well the thing is, there have been calculations done somewhere in the forums, and i'll go dig those up sometime.
But the general consensus is that Armor Pen > Crit Damage without IE but Crit Damage > Armor Pen when they have more than 100-150 armor, like rammus.
Armor Pen is stronger than Crit Chance when the opponent's armor is 20-60 AFTER REDUCTION.
This doesn't take into account the extra 50% damage from IE.
However, critical damage becomes stronger than critical chance once your crit rate passes 60-70%, which isn't until the end game anyways, and in addition, 15-20% crit chance with 200% damage in the beginning is better than 5-10% crit chance with, let's say, 215% damage in the beginning.
An armor pen/crit chance combo rune page would work in theory, but the 45 armor reduction from Black Cleaver will make your armor pen runes look like nothing anyways, and since minions don't have much armor to begin with, your armor pen wouldn't help you farm minis in the early-mid game, which is a must.
Thanks for the vote (:

Nice build btw have you tried using crit damage runes instead of crit chance, it seems to result in higher dps.
+1

Oh hey madmack, i've gotta check out your AP yi build soon,
I hear he's a beast mid if played right :)
I wasn't sure what to write so I kinda just wrote whatever,
hopefully the guide will get more polished soon though.
Thanks for the comment! We should play again sometime!

Lol, I've played with you, +1 for the title. Haven't read the build yet. We're lacking some good Trist builds on the site, work on this one hard and you could end up high on the charts :)