Heroes 5.5 WarGame Edition - Tactical Edition.

Because +100% to initiative is more effective than -60%.

What do you mean? It's the opposite...

For the same reason that increasing damage by an infinite amount is more effective than decreasing it by an infinite amount, i.e., to zero.

It depends on initiative and speed. Attacking first with infinite damage is better. But if you get hit first, it's better to nullify the enemy's attack. Although, I still don't understand the analogy.
In the case of initiative, slowing down and speeding up are almost symmetrical effects, if you only consider the troops. When heroes are taken into account, it's not quite the same, but it depends on which hero has better magic and better activated abilities.