Tiaral

I think we'll be attacking more often than defending.

Here's the thing.
In offensive missions, you can theoretically defend. To do this, you need to take a convenient position, engage in some minor raiding in the surrounding area, and wait for them to try to dislodge you. But technically, it's unlikely to be that simple.
In defensive operations, you can attack. Technically, it seems easier; you just need to move more actively than the enemy and take the initiative, without worrying about the fact that the territory is friendly and you can "hunker down." The risk is that in some situations, it might actually be much easier to "hunker down," or rather, in a fortress.
Of course, this is assuming that Reven hasn't created a system that won't allow for this, because I don't know.

It's for global battles, Ment.

Hmm. Well, considering that you were talking about "hero units"... Maybe. It depends, of course, on the amount of damage compared to the survivability of the units.
Then I would say, the first strike is better for a defensive strategy. Double for an offensive one.
And a question. If two opposing commanders both have the first strike ability, does it cancel each other out?
And if a double first strike is against a single first strike?