Iron Bane or Destruction, which is better?

[tibbs]

Iron Bane says all armor on all units become fragile and will be destroyed with a strong hit.

Destruction destroys armor in a 6 square area right out without relying on a strong blow to break the armor.

If say, you were being attacked by giants with strong armor, would it be better to cast iron bane and hope your mass of archers can deliver a "strong blow" or not take the chance and just have your mages cast destruction?

I guess I don't know what exactly a strong blow is. Can arrow hits be strong blows?

Thanks,
Chris