I have both level 70 Warrior and 70 Paladin, and for 5 man instances, Paladin is easier to manage for tanking.

Warrior:
Single Taunt is effective, and threat on multiple targets can be done, but keyboard intensive.
Tab, Sunder, Tab, Sunder, Tab, Sunder, Tab, Sunder.
Even if you set Sunder Focus, you'd still need to sunder a 3rd add.
Then on top of this, you will want to Shield Block ever 6 seconds; and reactionary Revenge.

Paladin:
With Avenging Shield, you snare and put threat on 3 mobs in a pull.
The button mashing is usually, SoR/Judge castsequence, Holy Shield, Consecrate as needed.
Holy Shield is 8 to 10 seconds spam.
Use Seal of Light/Wisdom and you replenish some health/mana.

Since you front load thread, you can DPS sooner with a paladin.
Add Salvation buffs on you party, then you're in a better position for 5-mans.

Gearing a paladin to tank heroics takes some research. Search "maintakadin" forum for more info.

My question is this

Should I carry on my initial plan to have Pally Tank, Priest Heal and Druid, Hunter and Mage DPS?

Definitely the Mage would be handy for 5-man, just raw AoE dps!

Priest Heal is nice enough, more complex that a Holy Pally healer. Shadow Priest is more button clicks. Channeling Mind Flay gets too intense when 3-boxing.

Druid Boomkin for more Mage damage & Paladin threat (spell dmg)!

Hunter feels odd, does any of his passive ability help dps as well?