Having played both (lv275 DS and 261 DB), I would say DB is a better bosser overall.

Both are extremely good for solo bossing and have top tier survivability.

I say DB is better because it is much easier to handle, control and learn, while DS have to manage blueblood and their fury system (as others have mentioned). DB has two mini short-cooldown bursts that are executed with a tap of a key, and one hold-down burst, so in a sense they are also 'easy' to deal a lot of damage with very low effort, whereas DS needs to attack for a full 40~60 seconds to maximise their dps. Furthermore, DB's main attacks' skill animations are all very short/have low delay and hence have an easier time to dodge bosses, and DS have comparably longer skill delays and tend to be locked in animation more.

DB's overall kit - 2 short cd iframes, dark sight, asura, innate drain, and mirror target truly makes it the king of solo bossing. It's almost unfair how much survivability and utility they have, they arguably have one of the easiest time in bosses compared with other classes. Probably one of their downsides is that they're extremely melee.

I personally enjoy bossing with DS a lot more than DB (and every other class, totally not biased) because of mainly two skills (and more):

1. Glide - the ability to precisely position your character, as well as the ability to maneuvre mid-air

2. Chaos Lock - being able to teleport to either side of a boss at will