Dual is better. Sure N Zap may be able to kill quicker at a closer range but if you are playing Dual you should not be playing close range, you should be playing mid range. Think of it not in terms of N Zap vs Dual but weapon catalog vs Dual or N Zap in battle. N Zaps may have a quick firing rate but there are other, more accurate, shorter ranged weapons with faster ttk that can take it out faster in close ranged play. Not that users cannot be successful with the gun but it takes more maneuvering and handling to do so then guns that either out-range it with slower ttk or guns of closer range with faster ttk. It is in this odd category of close-mid range, but not fully mid range nor really close range either.
In all situations N Zap needs to have the initiative on the enemy as well as map control (which granted echolocator does help). But with it's narrow firing path it is too easy to out maneuver the N Zap if the enemy is surrounded by their own ink. Honestly this is why NZap is getting movement buff later today with V2,2 update its one strength over most is its firing mobility. However it is honestly out-preformed by the tenatatek in the majority of situations due to ttek's accurate suction bomb in order to cut off N Zap's movement paths and more consistency and damage in shots fired. TTek also does the same with Dual close range but, as I said before, you should not be playing in ttek's range anyway. The N Zap's splat bomb is only useful when an enemy is stuck or distracted and not a very good spacer nor panic button for close range.