Added Chaos support gem vs Added Lightning?

"

DeviantLightning wrote:

Penetration doesn't scale Added Chaos, but doesn't have to because few enemies are both resistant to Chaos and Fire.

Aside from literally every Unique Boss, all of which have a shitload of Life to boot.

Bosses have some amount of baseline Resists below 75%; I have so far always assumed 50%. They can also have an appropriate Resist at a higher value but let's ignore that.
Versus 50% Resists, a Pen Gem grants 70% More Damage, plain and simple.

So, "any other Spell" you say?

Arc deals 382 average Damage
Added Chaos grants 201.5 * 0.5 = 100.75 base Damage
100/382 = 0.26 -> 26% More Damage
Worse than Lightning Penetration. Much worse against Uniques. Bad bad bad choice.

Firestorm 152.5 -- ACD 40.3 (26% - shit)
Ice Nova 237 -- ACD 201.5 (85% - very strong)
Ball Lightning 138 -- ACD 60.45 (43% - meh)
Incinerate 85 -- ACD 60.45 (71% - strong)
Flame Totem 76.5 -- ACD 40.3 (52% - acceptable)
Ice Spear 367.5 -- ACD 201.5 (55% - acceptable)
Storm Call 329 -- ACD 201.5 (61% - strong)
Spark 190.5 -- ACD 100.75 (52% - acceptable)

It's a strong Support, sure. It's far from universally applicable, especially on any build using the Heart of [Element] cluster due to the massive single-type Increased Damage and bonus Penetration.