Lightning Strike vs. Infernal Blow

What would you say is more viable in merciless?

In normal difficulty Lightning Strike seems to have better AoE damage (by far), while Infernal Blow seems to have superior single target damage while the AoE is only decent.

Does this change significantly with more +fire damage passives and with mobs who have more health in mercyless?

I am at the point where i have to decide if i go more into the direction of physical damage modifiers (for Lightning Strike) or elemental ones (for Infernal Blow, because the explosion is only affected by pure elemental modifiers).