4 Ret + DK or + Prot?

General question to the WoW multiboxers out there, for farming heroics why do people generally go 4 Pally's + a DK instead of + a Prot Pally? I've got a well geared DK tank and a well geared Pally tank and I'm leveling the 4 Pally goon squad, just curious as to which to use for the tank spot?

[lax]

DK is more for PVP. I run 5 paladins myself for PVE and it's great

[Ualaa]

Well a DK adds a lot more to the Rets then a fifth Paladin.
The Death Grip + Frost Snare are pvp oriented, but Horn of Winter and the Melee Haste are good everywhere.
In Cataclysm, two pallies are supposed to cover all of the Blessings, but each extra pally in another Aura.

But a Prot Paladin is a much better tank, for two reasons:
a) much easier to macro a strong and effective tanking sequence.
b) much stronger at lower levels of gearing.

If your focus is PvE, I'd run with a dedicated healer and a real tank.
If your focus is PvP, go with the DK and four rets for sure.
If you're interested in both, again go with the DK/Rets.