the rule is easy...all ranged units get attack first. all melee units get armour upgrade...exception ghouls where u do attack first. u don't use ghouls as a tank or meat wall as they are too weak. that's why u upgrade attack and not armour.
although if u go necros u are better upgrading both,
ghouls are only to save the day and serve bodies for skellies they don't stand a change against tauren or other sort of bigger melee. That's why maybe they have the frenzy upgrade which improves the speed of their attack and not armour.