Not sure which is a better choice for Merc in terms of survivability?

Ethereal

Vampire Gaze with 17 Dmg Reduction, or Non-Etheral

Vampire Gaze with 20 Dmg Reduction?

What are the defense values on both?

Is this for you or your merc?

If its for you, the non-

Eth is the winner as you can socket something other than a

Zod.

%Dmg reduction > Def in most cases imo. I think for VG its a difference of about 100-200 Def, not sure if that reduces the chance to get hit by 1%

It is for my merc.

I dont know the defense values for either but let's just assume they are equal.

Etheral - 17 Dmg Reduction / 15 Magic Dmg Reduction

OR

Non -Ethereal - 20 Dmg Reduction / 14 Magic Dmg Reduction

I would use the non-

Eth one for my merc with the information you have provided.
There are always other factors too. How much def does your merc already have? Melee merc or ranged merc? Is your merc's def boosted by another source?

Defiance from Exile or from a native aura? If the def is already really high an extra 100-200 on a

Helm will not out perform the extra DR from VG.

Graphene wrote: 4 hours ago

[..]
I dont know the defense values for either but let's just assume they are equal.
[..]

Gaze has a hard 100% ED. As such, the base

Grim Helm always gets 126, leaving you with 252 def non-

Eth or 378

Eth.

It only becomes an unknown if you upgrade it which has it reroll within the

Bone Visage range, giving you (roughly) a 50/50 shot of either increasing or actually decreasing the total defense on it.

All that said, def is nice and all and tends to still be theoretically relevant within a merc's normal def range but %ed probably still wins out overall.
Then again, given decent other merc gear, neither one will make any truly noticeable difference over the other. If I had to choose though, I'd go with the

Eth one. In part out of Principle (merc =

Eth!) but actually more so for the extra point of magic dmg reduction, seeing how elemental attacks tend to be more of a challenge for mercs than most physical stuff.