I've played both classes, so I'll try to do some outlining for you. Also, DPS is not the only factor in PvE; being able to survive while still dishing good dmg is very important.
~Katar can hit capped damage reduction and a large HP pool at the same time without spending a drop of AP or LP. Duelist needs to invest in AP/LP/Achievement fortification scrolls to reach this same conclusion.
~Katar has access to two damaging HOT skills that can be stacked 3x each with very short cooldowns, pretty much giving them permanent health regeneration in combat. This is very, very useful in dungeons.
~Katar as a main class has access to the 20% damage reduction from Inner Strength. This is one of the two biggest pros to the class alongside the stackable HOTs.
~Katar as a main class gains a 15% HP boost as the starting class passive in their envoy. This means it will not take a ton of effort to build up a strong HP pool on top of their already great defensive/survivability capabilities mentioned above.
~Spammable stun! With a tiny cooldown!
~It does not have access to the damaging DOTs that Duelist has. Shadow Strike, one of the stackable HOTs, is a DOT that stacks 3x but it is its only DOT skill.
~It does not have passive HP leech like Duelist does with Bat Fang. So unless you get lucky with Nocturnal prefixes, you will rely on Inner Strength's tiny healing pool and your HOTs for your main healing.
~Bat Fang + Balisong = gg with mobbing (if your squishy body doesn't turn into food for the mobs first)
~Access to a great pool of DOT moves for steady and quick DPS capabilities
~Balisong = a great AOE with a somewhat short cooldown that has decent range and also leeches HP back to you from every mob it hits/damages
~It is nowhere near as survivable as other classes, but specifically compared to Katar, unless you build heavily into both defense and evasion
~No natural stuns
~Its DPS is not as bursty as Katar's is. Overall Duelist has better DPS yes, but this is in part because of DOTs. In terms of pure skill damage and DPS capability, ignoring all DOTs from both classes, Katar is burstier and heavier
***This is all in general about the classes, talking about the average player and not "the best of the best" of anything. ***