The discussion of which of these two weapons is best comes up frequently in the WoW warrior forums. The general argument comes down to this:
Mitigation and ThreatEdit
The King's Defender is the better choice for a warrior tank that is aiming to maximize on his/her Effective Health and increase general threat production via the bonus to hit found on the King's Defender and the higher base DPS.
While in general a tank will be better served by selecting gear to maximize effective health, some fights are better suited to higher levels of avoidance (Bosses that hit very hard and frequently such as Prince Malchezaar).
For avoidance situations The Sun Eater greatly outperforms the King's Defender by providing 1% additional dodge and only 6 less stamina. The Sun Eater is easily the tank's choice of weapon before entering Karazhan and can be used into Serpentshrine Cavern in the event that King's Defender proves difficult to obtain.
In The Long RunEdit
Acquiring both weapons and using them as necessary based on the requirements of the fight may serve the tank best.
Warriors will find the , a drop from The Lurker Below in Serpentshrine Cavern to be an upgrade from either weapon. The threat increases provided by the weapon's DPS coupled with the expertise rating makes this weapon far better suited to the high threat demands of a progressive 25 man raid. The loss of mitigation stats on this weapon are compensated for by the equivalent armor pieces found in Tier 5 content.
Another option for upgrading is the , a drop from Zul'jin in Zul'Aman. The Cleaver is designed to be on a similar tier with the Mallet but with a higher preference towards mitigation, though the gem slot allows tanks to make a choice regarding customization.