Tell me if I got this right:
RtK mythic has typical loot drop chances and saves a lockout.
RtK heroic has, as wowhead states, extremely low loot drop chances (1-digit) and saves no lockout; can be looted as many times as you want.
The few runs I did seem to confirm this. (I am trying to farm the witch hat, but with the Wikket version only being up every three weeks, this seems to take a long time with two cloth chars and a drop chance of 3%.)