It seems that First Strike is much better than any of the other upgrades for an offensive unit.

Context, young padawan, it all depends on the context.

Like what jkp said. If I know a unit is simply going to sit in my city and defend then I give it a city defend bonus. If it's going to be out fighting in the wild then the first strike might be advisable.

well I think first strike is cool for defense also because if I have 1 or 2 in my city and the enemy is not at city but in range of my unit then - a good offensive strike helps the defense.