% damage reduction is multiplicative, not additive.
So, if you have a Pally and a Warrior in the same gear (which they could be, though they may gem and enchant with different priorities, I'm more inclined to stack stam on my pally for the spell power added value and Ardent Defender), talents and spells make the difference, right?
From possible talents:
Warriors = +5% dodge, +5% parry, +10% armor from gear, +30% shield block value, -6% spell damage taken, +6% Stam/Str, 6 expertise, 30% chance for double block
Paladins = +5% dodge, +5% parry, +10% armor from gear, +30% shield block value, -6% spell damage taken, +15% Strm +12% Stam, 6 expertise, -3% all damage taken (Shield of the Templar), 10% chance on attack made against you (regardless of outcome) to increase block chance by 30% for 10 sec OR 5 blocks, below 35% health all damage taken reduced by 30%,
Warrior (Warrior only):
Shield Block = 100% block chance, double block value for 10 sec, 40 sec CD (talented)
Shield Wall = -60% all damage taken for 12 sec, 4 min CD (talented)
Defensive Stance = -10% all damage taken
Warrior (always on Warrior, but can be shared):
Thunder Clap = AOE, no target limit, 20% melee attack speed reduction, 30 sec duration, 6 sec CD.
Demoralizing Shout = -410 AP to all enemies in 10 yrds (at 80, untalented)
Commanding Shout = +1855 health to all friends in 20 yrds for 2 min (at 80, untalented)
Paladin (Pally only):
Holy Shield = +30% Block chance, 10 sec duration OR 8 blocks, 8 sec CD
Divine Protection = -50% all damage taken, 12 sec duration, 4 min CD (talented)
(I'm assuming that you won't HoP or Divine Shield yourself while tanking)
Imp Righteous Fury = -6% all damage taken
Paladin (always on Pally but can be shared):
Judgments of the Just = judged targets have 20% reduced melee attack speed (20 sec duration, 10 sec CD untalented)
Devotion Aura = (at 80) +1800 armor, +6% to incoming heals
Blessing of Sanctuary = -3% all damage taken, dodge/block/parry returns mana/rage/RP
Resist Auras = +130 resistance to Fire/Frost/Shadow
Blessing of Kings = +10% to all stats (for these tanks, Str/Stam/Agi for survival)
So, both tanks have the chance for a lot of the same buffs from fundamentals, so we'll cross out commonalities for a moment:
Warriors = +6% Str, +6% Stam, 30% chance for double blocks
Pallies = +15% Str, +12% Stam, -3% all damage taken, when health drops below 35% all damage taken is reduced by 30% (functionally more stamina or more value from Stam), 10% of attacks increase block chance by 30% for 10 sec or 5 blocks
Assuming the two are not in the same raid, from specials:
Warriors can double block value and block everything for 10 sec in 40, they have 10% static damage reduction, their Shield Wall is 10% more damage reduction (though personally I don't talent it down to 4 min CD), and they can melee slow everything they're tanking. They can also apply a static AP reduction.
Paladins get 1800 more armor (not the biggest deal) with 6% increased heals (bigger deal), stack up 9% passive, universal damage reduction, they can keep Holy Shield up almost 100% unless they're tanking enough to get more than 8 blocks in 8 sec (never the case on a single target) which is worth 30% more blocks. Between Redoubt and Holy Shield Pallies will block a lot more than warriors but they won't block as much when they do. I haven't talented BoK since I can't give that to myself AND BoS, though I could see an argument for trading 15% Str, 3% damage reduction, and the mana regen for 10% Str, Stam, and Agi for survival value (personally I just love the mana refund too much).
Put them in the same raid and the pally can give the warrior 1800 more armor and 3% damage reduction (with the silly powerful rage generation), and the warrior can give the pally 1855 more health, AP reduction on the boss, and the balance of Tclap and JotJ become moot. Where pallies will have functionally higher block chances, warriors will often block more with their blocks.
I don't have time to do the math now, but I'm going to go out on a limb and guess that pallies have better passive survival buffing from talents without accounting for other conditions. That being said, where they can, warriors being able to Concussive Blow, Shockwave, Spell Reflect, Shield Bash, and Charge stun, allows them to severly limit their liability in a different way. Against things that can't be silenced or stunned though (raid boss?) these things have less value.
I think the fine print here is that the two will like the same pieces of gear, but they will buff them differently and that difference will put them more on par. As it stands I'd think if you took a prot pally and prot warrior, and put them in identical gear, the Prot pally would be a more durable boss tank, and the warrior would be a stronger tank on stunnable trash.
If you want to discuss threat, it's a lot stickier territory since that has a lot more to do with player skill and they're completely different mechanics, whereas survival has become more often a matter of knowing when to use your emergency tools.
There's also the matter of group protection. Warriors get Intervene, Vigilance, Imp Spell Reflect, and stuns (where usable), and Pallies get Hands of Protection, Salvation, and Sacrifice (and Freedom), AOE taunt, Hammer of Justice (where usable).
Really, I think if you're looking to stack your tanking setup a smart prot pally or warrior with a well put together dps set could amplify your main tank and still compete with the dps in the raid (though they might not be on par at 80 in higher levels of gear). Furthermore, with dual specs, having a warrior or pally in the raid with a tank off-spec may be able to give out certain buffs (BoS comes to mind in a big way) to amp your tank then switch to a full-on dps spec.
Either way I'm loving both my tanks and the ability to solo/dps well and even dps 5-mans while not tanking while actually contributing, not just being carried. I'm loving the 3.0 world.