There have been many threads on this topic. Use Google (not the forum search function) to search for "spiral knights leviathan cold iron vanquisher" (without the quotes), and you should find them. My own answer is here.
Cold Iron Vanquisher VS Leviathan Blade
There's pretty much never a reason to use CIV. Elemental weapons always outshine it, and if you're fighting gremlins/beasts, that's why you have TWO weapon slots.
Although both Levi and CIV are weak...Gunners and bombers use them because they deal normal damage.
Levi`s charge is better if you play solo because if you`re in party it can throw enemies to other party members,while CIV has a "friendly" charge (poor knockback).
While Levi`s charge hits the enemy 1 time (if it`s not cornered), CIV can hit 2-3 times with its charge,which means you will kill the enemies faster.
As I said-both are weak :( I choosed CIV because its better to deal good damage vs Undead (its NORMAL dmg weapon) and low damage to all other monsters than to deal average (low,compared to other swords) dmg to all enemies...
> Include other weapons in comparisons between weapons.
Leviathan is "Stronger". It has a higher base damage.
Cold iron is more "Situational." It has a lower base damage, but the charge attack is team friendly and may hit more than once on non-stationary enemies, effectively making it a better DPS weapon if you are charge reliant.