Which stat is more valuable to your hero will depend on its class.
1 STR = 1 Armour
1 DEX = 1 Armour
10 INT = 1 All Resist
Strength-based heroes (Barbarians, Crusaders) have STR as their main stat, so they have a naturally high Armour value. This is why they take All Resist on armour pieces and use Diamonds in their Chest / Legs, as the All Resist provides a bigger increase to their overall damage mitigation than adding Armour would.
Dexterity-based heroes (Demon Hunters, Monks) have DEX as their main stat, so they have a naturally high Armour value. This is why they take All Resist on armour pieces and use Diamonds in their Chest / Legs, as the All Resist provides a bigger increase to their overall damage mitigation than adding Armour would.
Intelligence-based heroes (Necromancers, Witch Doctors, Wizards) have INT as their main stat, so they have a naturally high All Resist value. This is why they take Armour on armour pieces and may use Rubies or Emeralds in their Chest / Legs, as the Armour provides a bigger increase to their overall damage mitigation than adding All Resist would.