It depends from the average physical damage of a monster and can be derived from formulas provided by tibia.wikia.com for damage reduction:
Minimum damage reduction = totalArmor * 0.475
Maximum damage reduction = totalArmor * 0.95 - 1
So now lets take stats for both legs you've mentioned:
Zao - armor 8, phs dmg reduction 2%
Dwarven - armor 7, phs dmg reduction 3%
So by using the formula minimum and maximum damage reduction will be as follows:
Zao: min = 3.8 | max = 6.6
Dwarven: min = 3.465 | max = 5.65
Now we can use those values to calculate the damage from a monster which is derived from the formula:
(attack of the creature) - (defense) * (shielding) * (defense factor / 100) - (attack of the creature / 100) * (totall armor) = average damage
Now that you have average damage of the monster for each of legs you can remove the 2% and 3% from it and see which legs are better for you.