Empower vs Penetration?

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Nasreth wrote:

Empower is better for your tooltip dps and it's better if you're dealing lots of different elemental damage types rather than just 1.

That's also relative and we could say it's strictly true only for builds that convert damage or use percentage adds like cold to fire builds, for instance. If you got a lot of flat added damage from auras or support gems that damage won't be affected by empower.