Just wrote up a very small bit of math for the WoW-US forums, was a bit surprised to see the Armor Enchant scaling line up so well on level 73 creatures (it's a couple decimals off).
An additional 120 Armor on 10,000 Health will allow you to take 100 more raw damage;
An additional 120 Armor on 15,000 Health will allow you to take 150 more raw damage;
An additional 120 Armor on 20,000 Health will allow you to take 200 more raw damage.
Not to say Dodge or Agility are bad enchants for cloak; those are the raw numbers for Armor so you are aware of just what you are getting.
Dodge to cloak represents about ~0.63% Avoidance. Armor to Cloak represents a ~1% increase to Effective Health and a ~0.99% reduction to mitigated incoming damage from a level 73 creature.
For me, I placed +120 Armor to Gilded Thorium as that's my primary cloak and also my high Armor/Stam cloak. I have Dodge on my Devilshark and switch that in whenever I want a bit more Avoidance and a very slight reduction to Crushing Blows.