It all depends on what you want.

A Paladin tank is far and away a better boxing option then a DK, as far as tanking goes.
Single and multi-target pulls.
Passive threat on everyone, just for blocking.
Much better survivability.
Much easier to play at close to maximum efficiency, while boxed.
The DK would be tied with a warrior (for 3rd and 4th place, on my tanking list behind Pally 1st and Druid 2nd); but they do get you the nice damage buff from the tank...

But you have to balance that against....
A druid only applies Earth & Moon, to targets hit by Wrath or Starfire (not AoE).
A warlock only applies Curse of the Elements to single targets which are hit by the spell.
So both caster options can have it on multiple targets (for the duration anyway), but need to cast on one at a time.

I have a druid on a 5x caster team... druids click macro includes 1x Wrath as an opener, then IS/MF for bosses or straight to SF/SF/SF or Wr/Wr/Wr/Wr stings, which alternate between one and then other.
The warlock does either Curse of the Elements on bosses as the first click, or just spams Seed of Corruption.
It works well for both teams.
The 13% only applies really against bosses, as trash dies so fast it doesn't matter.
Of course trash might be tougher in other content, but for my teams and what I do, it only matters on bosses.