Iapetes claims whirlwind should be a strong single-target attack, in addition to the AoE. In other words, not 50% damage. I agree.
Whirlwind should do more damage on average. I suggest that whirlwind emphasize the berserking nature of warriors. Make it unable to miss, be dodged, or parried. It would always hit.
Between 0 and 20 targets.