sins and conceal opers can go back in forth dueling but sins have a stronger skill set.
better question would be, which sin/shadow spec is the best for dueling?
Against lolsmash maras/juggs, hybrid survivability/useless spec like 24/17/0 is best. You'll take the lolsmashes for maybe 3200 instead of 4500 with the aoe damage reduction, you'll have the armor rating from darkness, but your only real big attack will be maul. With this spec, you'll be able to kill lolsmashers if you time all your cds well and have a good rotation, which largely consists of thrashes, energized shocks and mauls. Without the aoe damage reduction, you're going to be at 50% hp before you know it or can react in time, speaking from personal experience (especially if you have full min/maxed gear like I do and ~18k hp). You won't pull big pvp numbers with this spec, but you'll survive and win duels if you're solo guarding a node.
Every other class, best dueling spec is: 27/0/14, you'll be able to duel and wipe out every other class with that spec. Deathfield instead of wither will also give you more damage against vanguards/pts when they have their shield up, but even in those cases, usually best to cloak out their their shield wears off, then go back in to finish them off. spam deathfield whenever it's off cd, thrash a bunch (I usually get 1700-1800 thrashes), shock a bunch, then when you have 3 stacks of harnessed darkness, electrocute, recklessness, force lightning. Win.
You could probably use the 24/17/0 spec in general, but you're going to have far less overall damage in each map and it's a lot more reliant on mauls critting, which they don't often (I'll get maybe 1 5800 maul, then 4-5 1600-2400 mauls in a row). Also easiest to use this spec if you use the set bonus for 50 more force, since it doesn't hurt you as much if you spam maul even without the proc, whereas with just 110 force vs 160, you're going to run out of force fast.
but yeah, 24/17/0 against lolsmashers, 27/0/14 or 27/1/13 against everything else. You'll win duels against any class with those 2 specs 99% of the time. Just be careful when dueling operatives, or you can get stunlocked and lose 50% or more of your health in a few seconds.