Which alternate upgrade for Inferno do you like the most?

So, if the counterattack is successful, the demons will deal six times the damage? And if they attack warmongers, it will be 12 times?! Hmm... Or am I mistaken?

I think you're confusing things. In Heroes, the damage doesn't increase by n times, but rather by 100%. That is, a regular creature's counterattack will be 100% (base damage) + 200% (the demon ability's bonus). With luck, it's another +100%, and warmongers with luck get +200%. In total, the maximum damage can be increased by 400% or 5 times.