I would think a bear tank would be easier then a deathknight tank, at least looking at warcraft from the perspective of the current expansion. But a click castsequence certainly works for a DK.

You already have a druid healer in your proposed groups, so the buffs are already present.

I like the idea of essentially two five man groups: Tank, Healer and 3x DPS. And one of your DPS can dual-spec to be the 3rd healer.

From a loot perspective:
- Plate: Pally Tank, possible DK Tank.
- Mail: Shaman x2, Hunter.
- Leather: Possible Bear, Druid Healer, Druid Boomkin.
- Cloth: Priest, Warlock x2

The composition is predominantly caster based, I don't see a huge difference between a Bear and a DK. If you go with the DK, you end up with 2x Plate and 2x Leather users. But the Bear would be the only user of melee leather anyway.

Possibly change one of the Warlocks to a Mage, for Arcane Intellect and an easier CC option. Plus portals and summoned food.