If a creature hits an average of 100 damage every hit, Demon Helmet will reduce the damage with a minimum of 4.8 and a maximum of 8.5 while Zaoan Helmet will reduce it with a minimum of 4.3 and maximum of 7.6 with an additional of 5%.
In the end, Zaoan Helmet will reduce the damage by 9min and 12max compared to Demon helmet with 5min and 9max.
Dwarven Helmet only reduces the damage by 2.9min and 5.7max with an additional of 2% which goes reducing 4.8min and 7.6max. Keep in mind that "100 damage" was used for the calculation. Zaoan Helmet price has decrease since Elite Draken Helmet was implemented, because it was mostly used by Paladins.