Currently I would say paladin. You’ve got a healing spec which dks do not.
You also have utility. Hands, offheals, cc, stuns, while dk has grip and a stun if you talent for it.
Dks are struggling alot currently in the damage side of things too, frost can do nice cleave burst with BoS, unholy can decimate massive groups with the right set up. Both are good in their very small niches.
Unholy is doing well in pvp atm I believe.
Just to clarify, the trait is festermight, it caps at 20 stacks, and it’s presence or absence basically determines if an azerite piece is usable. Unholy’s mass aoe is more about the way bursting sores scales with targets. Each wound burst does splash damage when you use that talent, so it scales up very fast, again capping at 20 (I believe it’s 20 targets, could be 10), but each target dealing equal damage to nearby targets.
To produce the big unholy aoe, you need a tank who can pull enough mobs, then you use your ghoul empowered to hope it puts wounds on alot of targets (30% chance per hit when using infected claws talent), you can apply some with festering strike but then you need to wait for runes to come back up, then drop death and decay, pray the tank doesn’t move the pack away, then you can start scourge striking and making things explode.
Or pop wings and throw out divine storms or just do standard havoc stuff.