As far as I remember, 30% offense is a bonus to base damage, which as a result may add significantly less to the total damage. I haven't tested it myself yet, but it turns out that with high enemy defense, the bonus could be more than 30%.

Let me clarify :)
If the attacking unit's attack is greater than the targeted unit's defense then

U=u*k+u*30%(if expert offense)

where U-damage dealt by the unit
u-unit's personal damage.
k-increasing coefficient equal to (attack minus defense) *5%

That is, we see that with higher attack, offense only increases base damage (as does shooting, for instance)

For example, a Barbarian Angel with 6 attack and expert off hits a unit with 6 defense. Angel's Damage=50*(1+20*0,05)+5*0,3=115 (instead of 130 if offense multiplied everything by 1.3)

If the attacking unit's attack is less than or equal to the targeted unit's defense then

U=u*k*1,3(if expert offense)

where U-damage dealt by the unit
u-unit's personal damage.
k-decreasing coefficient equal to (100%-defense minus attack *2,5%),
in this case, offense is compared with defense.

IMHO: defense is better than offense. :) arguments
1) works against everything (shooters, melee)
2) in Dragon Utopia and forests they kill us first and then we kill them (mostly); if there is no defense, then offense might not be needed :p
3) Offense with higher attack doesn't add 30%, but less (see formulas above)
4) In TE armored is 20%, meaning everything that hits you is *0,8. And that is good)

PS I consider Armored the best skill in TE. I always take it regardless of what else is offered