First, let's clear up a misconception.
Armor Penetration gives you a constant %damage bonus, regardless of how much armor the enemy has see here.
It doesn't matter if your enemy has 300 armor or 15 -- penetrating 10 of that will have the same effect on total DPS increase.
That said, let's take a look at why Armor Penetration is preferred, even with its linearity.
Attack Speed is easy to acquire via items.
Armor Penetration is not.
You can more than match the effects of attack speed runes with a single dagger - 420 gold.
The cheapest item with Armor Penetration is the Brutalizer at 1337 total gold. By this metric alone, Armor penetration runes are worth approximately three times that of attack speed runes.
It is easier to use items to stack Attack Speed over Armor Penetration -- but to get the highest DPS, you want to stack both.
Given that the amount of Armor Penetration available via items is strictly limited, if one's goal is to achieve maximal values for both Armor Penetration and Attack Speed, runes for Armor Penetration are much better than that of Attack Speed.
For early Game your best bet would be:
Red: Armor Pen
Yellow: Flat Armor
Blue: (Your choice, I normally base this one depending on who I am against, Magic Resist, Attack Speed, Attack Damage, Cooldown Reduction or Mana Regen are all good choices.)
Quints: Attack Damage or Armor Pen (I prefer damage, I find you get enough armor pen with your masteries and reds to be able to use quints as extra damage source, if you are having trouble with armor or they are stacking armor just buy an armor pen item in late game.)
That will give you a good rounded opening with the ability to be very aggressive (because of your armor) if you find your opponent leaving themselves open or if they over extend.
Best Answer
I have no hard numbers but I will explain the reasoning for the choices.
My recommendation: Flat attack speed runes in every slot.
The reasoning. His passive scales perfectly with AS and most Warwick builds are based around on hit effects. (Wits end, his passive, Bloodrazor). AS helps him clear camps faster, gives him more sustain and scales extremely well late game.
Armour pen runes - These are a no go in my opinion. 10 Armour pen is enough for jungle creeps to have 0 armour, however Warwick doesn't scale very well with these runes. They may be good for optimal jungle clearing time, however I wouldn't recommend them due to their usefulness falling off pretty hard late game.
Attack damage runes - Warwick doesn't scale so well with AD, so as above, I would not recommend them.
Flat armour seals - I would recommend these on every jungler, except for Warwick. He has build in sustain. You will want to build a wiggles anyway, so the cloth armour will be more than enough for you.
Magic pen runes - These are very strong for lanewick and scale well late game. However, they are not optimal for faster jungle clearing times.
Magic resistance glyphs - These are an option instead of attack speed glyphs. This decision makes your clear time slower. Warwick tend to end up with a fair amount of magic resist through most builds anyway, so I'd say AS runes are preferable unless you are against an extremely AP heavy team.
Movement speed Quints - These are another option for WW. He will be able to travel between camps faster as well as help his ganks. His bloodscent makes these a bit redundant later on in the game as you will probably be moving faster than everyone else already anyway. Attack speed quints are probably still preferable due to sustain they give.