General Discussioneffective HP

You don't get any HP from Assault Cuirass, just armor, aura armor, aura minus armor, and good attack speed.

But if you're talking about which of the two items are more typically built on those heroes:

Juggernaut- Skadi
Phantom Assassin- Assault Cuirass
Faceless Void- I can picture him building AC later on if no one's building it and your team needs it, but Skadi, not at all.

http://devilesk.com/dota2/apps/hero-calculator/

paste your build and see the difference

or just do the math yourself it's not that hard

against what

against physical damage dealers

thank you for your replay guys

On lvl 1 this is what you get:

SKADI = Physical EHP: 1893

AC = Physical EHP: 1271

On lvl 6 this is what you get:

Skadi Physical EHP: 2301

Ac = Physical EHP: 1730

Also dont think about stuff like EHP unless your 5k+ ( even most 5k dont know that Armour stacks endlessly equally )

In general each point of armour increases your effective HP against physical damage by roughly 6%.

Skadi provides 725 HP and 3.57 armour.
AC provides 15 armour.

If your HP pool is very small then armour is far less effective, since you're adding 6% of a small number. Therefore its more efficient to buy flat HP. If your HP pool is large, then adding 6% to it by getting armour is more efficient then adding more flat HP. Like Sir showed, at levels 1 and 6 where your HP pool is small its better to have a Skadi. As you get more HP the armour from AC becomes more and more efficient, eventually reaching a point where it increases your effective HP against Physical damage more then the Skadi.

If you want to calculate that point the equation should be roughly:

(x + 725)*(1 + (3.57*0.06)) = x*(1 + (15*0.06)) where x is your flat HP before buying either item.
The term on the left gives the effective HP from a skadi while the term on the right gives the effective HP from AC. Solve for x to find the HP at which both will be equally efficient. For any number higher then that, its better to get AC and for any number lower, skadi is better. I don't have my calculator around, otherwise I would solve for x myself.

Note: That equation might not be entirely accurate since I'm not sure quite how armour stacks.
Note 2: Obviously this only lets you know which will give you more effective physical HP. Skadi is always better against magic and pure damage and in game you should consider the other things the items provide before deciding which one to buy.

I didn't calculate it but a rough general idea is under 2k HP skadi more physical ehp and over 2k ac more ehp vs physical . So unless you have a lot of Hp skadi generally makes you tankier since it works vs magical and pure too .