This has all been addressed before, but the OPS has oversimplied, which makes things a bit incorrect. Here's some better details:

Armor Pen: 29 armor pen with runes, 6 with masteries. 35 total. This can be pushed up to 50 with a brutalizer. that's about as high as it can go. period.

Crit chance: 32.7 crit chance from runes, 3 from masteries. 35.7 total. This can be maxed out to 100 easily, with any number of builds.

At face value, these two values appear about the same, but that's an illusion for a number of reasons:

1.) armor pen+crit chance stack. That means that if you've got 30% crit chance matched up with 30 armor pen, you're going to do more damage than if you had 60% crit chance. Here's an example:

100 damage+60% crit chance makes 160 damage on average. If the opponent has 75 armor, you only deliver 91 of that.

100 damage+30% crit chance and 50 armor pen makes only 130 base damage, but vs an opponent with 75 armor, you're hitting for 104, a significant improvement over the all-crit option. lets explore further though...

lets assume you go all crit runs and get a brutalizer just to help out. You've maxed out at 100% crits and 18 armor pen (brut+masteries)

100 damage base turns into 200 damage with crits, your opponent has 75 armor that you've reduced to 57. You now hit for 127.

...but now let's say you spec'd for armor pen. You're 33% less likely to crit due to your different rune choices, but you have 50 armor pen and 67% crit chance.

You hit for 100, averaged out to 167 with crits. your opponent with 75 armor is reduced to 25. You hit for

133, a slight advantage.

There is one more thing to consider though: the armor pen build can still get full crit chance. Let's do that:

100 damage crits to 200, with 50 armor pen against 75 armor: end result hits for 160... a large advantage.

it costs about 1,700g to gain the extra 30% crits. to increase damage by 30% through AD items for a late-game carry, you'd be spending 4k+ for equivalent damage.

__Conclusion:__

Although there are some points where crit chance will offer modest (1-5%) benefit over armor penetration, the gains in those scenarios are small compared to the double-digit percentage increases that one can achieve through armor penetration. Unquestionably, by endgame, armor pen runes will be significantly better than crit chance runes due the abundance of crit chance items and shortage of armor pen items.