So when it comes to tanking, both Paladins and Death Knights are effective. They both have their strengths and weaknesses, and one strength they share is that of AoE tanking. Granted with Crowd Control AoE tanking is not going to be used every pull, but there are some pulls where it just needs to be used, or possibly just to liven things up! (I'm not telling you to pull a Leeroy :D) Here I'll be comparing the differences, high points, and low points, of each Class.

DEATH KNIGHTS

Death and Decay - A great ability, this generates incredible amounts of threat as well as dealing descent damage. You'll generally want to lay this down when the mobs are in place and they won't be running out of it, as it has a 30 second cooldown and only lasts for 10. Certainly one of the best abilities in your arsenal. I would consider slotting in a Glyph of Death and Decay.

Pestilence - Pestilence is currently used to spread your diseases, with a bit of damage added when it hits. In patch 3.1 Pestilence itself causes no damage, only applies your diseases to the enemies nearest your target. Glyph of Pestilence is a give or take, increasing the radius of it by 5 yards can end you up in a good or bad situation.

Blood Boil - A pure damage dealing ability, it deals X damage to all targets within 30 yards that have diseases on them, however, in 3.1 it will deal damage to all targets within 30 yards even if they don't have a disease on them. It will deal extra damage for each disease on the targets. This ability should generally be used after Pestilence.

Howling Blast - Deals X amount of damage to all enemies within 10 yards of your target, but it deals double the damage if they are infected with Frost Fever. Once again you should try to use this mainly after you've used Pestilence.

Unholy Blight - Deals X amount of damage to all enemies within 10 yards of you every second for 20 seconds. Once again, another GREAT ability if you have it. It deals amazing damage, which in turn causes high threat.

That about sums up the AoE abilities of the Death Knight. Next I'll talk about the Paladin AoE abilities.

PALADINS

Consecration - The Paladins main AoE ability. It consecrates the ground beneath you, damaging anyone that walks upon it. The effect lasts 8 seconds and the cooldown of the ability itself is also 8 seconds. This combined with Righteous Fury can produce massive amounts of threat.

Avenger's Shield - A range ability from the Protection tree that hits up to 3 targets. Extremely useful for pulls as Paladins don't have any other source of ranged attack.

Hammer of the Righteous - A great melee range AoE attack. This ability hits your target, plus 2 others near it for 4 times your main hand weapon damage per second as Holy damage.

Holy Wrath - A situational attack, this will certainly help you out a lot when you're tanking Naxxramas but otherwise you will hardly be using it. It sends bolts of holy power in all directions, causing X amount of Holy damage and stunning all Undead and Demon targets within 10 yds for 3 sec.

These are the Paladin AoE abilities you will most likely be using if you are ever called upon to do so.

CONCLUSION

Death Knights certainly have more abilities in their arsenal that will do full, AoE Damage to all targets around them. Most of the Paladins attacks hit only around 3 targets, but their main AoE ability can be used repeatedly and generates high amounts of threat. All in all, bring the player not the class. If you have 2 Players of the exact same skill and gear level, one a Death Knight, one a Paladin, I'd say they'd be about equal. It may not be the answer you're looking for but it's what I'm willing to give and what I think makes the most sense. Good luck!

~Aenorn