Which is better, attacking or defending?

Yes, but in the third game, attack increases damage by 30%, while here it's only 15%.
And defense is the opposite, but in the third game, it also protects against ranged attacks.

Hmm, that's true. But in the fifth game, there's a useful secondary skill – dodge. I've never actually tried to calculate the damage from direct attacks. I think it's more useful to develop secondary skills, especially "Battle to the Last": combined with regeneration, you can use five Imps to wipe out legions of neutrals.