Well the cooldown could viably be replaced with mana regen, though right now, that's not exactly my top priority as cooldown still helps, at least a little (though you're right wih her, it's prob not the best choice).
The reason why armor pen may be the better choice with her is b/c her ability can bring them into negative armor, which give bonus damage. so if the hero has 34 armor, I can actually bring them to -10 with ravage which gives about an 35% extra damage every hit. (that includes the 20% or so not being reduced from armor.
So looking at the math, if I hit for 80, I am getting a bonus 28 damage a hit from armor pen.
10% chance to crit gives me a bonus 80 (1/10 times) which is a bonus 8 per hit on average.
The question is, is the burst from crit worth it? Also, since my crit chance is low, does that raise the value of the crit chance runes? i mean 8.3% is a lot when my crit levels only ever get to be about 20-25. (33% more chance to crit)