<p>They are probably the same. The King's Sword was a one-hit-kill on every enemy it came across except Zurvan and the Dark Prince. The Water Sword needed 3 hits to kill Brutes, and a lot more to kill the Dahaka. As an aside, The Sword of the Enlightened Warrior was a one-hit-kill on every enemy it came across because it never fought a boss. </p>

<p>I would say the King Sword is on the fact that it didn't require to be made of Water in some form (in which Sand Monsters have a strong aversion too) to get quick kills (though it is a durable blade itself). Also, it's one of the only weapons present in the SoT franchise that's not a relic of time or has water but kills Sands Monsters in one hit </p>