NHF – new heroes for Heroes V 3.1.

Agrail's special ability (buff) and Railag's (debuff)
What if they are not used against each other?

If they are not used against each other, it becomes more complicated. As I mentioned, the guys from Shio aren't the most proactive, so a bonus to initiative might not be as effective for them. It would work better on the Fortress.
Also, the initiative debuff should work well for heroes with strong magic. Conversely, the initiative buff works better against such heroes (and generally works more effectively if the enemy actually has a hero, although how important that is is a question).
Overall, there are many nuances here, and it's difficult to assess all of this definitively, at least without numerous (very numerous) tests. So, I would consider these abilities to be roughly equal (assuming the mathematical effects are the same).