Elemental Damage vs Skill Damage

Recently a quiver dropped that could have either it's Elemental Damage% or it's Skill Damage% re-rolled. Both of the naturally rolled properties were not used by my build. I need to re-roll one of them but which one will give me the biggest damage boost?

My build is an Impale Demon Hunter, which really only has one the active damage skill of Impale (there is no generator used). Assume the Impale rune chosen makes the skill use Cold.

I am assuming that the Elemental Damage% will be better simply due to the higher upper limit maximum [15% - 20%] compared to the Skill Damage [10% - 15%]. I also guess that the Skill Damage% property would be better if the gear is used across multiple different builds. That way the rune on the Impale skill could change (thus the element could change) while still retaining the +15% damage.

Normally I would just re-roll a perfect roll for each and compare the difference, choosing the highest damage increase. However since in this case I can only re-roll one of these knowing ahead of time is required.

Questions:

1. Is Elemental Damage% better than Skill Damage% for pure damage? Are there other factors that I am missing here? (i.e, not listed above)
2. Does this change if I already have existing +15% Impale Skill Damage and +20% Cold Damage? (Therefore the choice is between +30% Skill Damage vs +40% Elemental Damage).

While this question uses Demon Hunter as an example I would assume that the underlying mechanics would be the same for every character, if not please explain why.

Solution 1:

+20% from Elemental% will give you more DPS than +15% from Skill%.

Because both are of the same type +X% Damage and are added before applying other types (Attack Speed for example). So if you have on different items +15% Cold, +15% Skill, effectively you will have +30% Damage.

However, I would recommend to throw this item away, as it's not perfectly rolled for your build - you really want to have both +% your elemental damage and +% your main skill.

For example, check out this Cold Impale Demon Hunter build:

Holy Point Shot quiver is a mandatory piece that practically triples your damage output. An ideal roll would include your preferred elemental damage and Impale % alongside the usual high Dexterity, Attack Speed and Crit Chance.

Also, stats priority for this slot:

1. Dexterity
2. Critical Hit Chance
3. Cold %
4. Attack Speed
5. Impale %

That means %Skill on this slot have smallest impact (according to authors of that guide).

Also, you can confirm that by constructing your item in tools like D3Planner.