So the thought occurred to me, let's figure out just how well the 2k extra HP and 4% more physical reduction Warriors get compares to Ardent Defender. Math inc, pardon my dust:
Some base numbers -
Warrior max HP: 20k
2k HP gap puts the Paladin's HP at 18k
Ardent Defender threshold at 18k is 6300.
Let's start with an incoming hit of 5k. In the absolute worst case scenario that the Warrior can survive, he would be sitting at 5001 hp. So, Warrior takes 5000 damage, survives with 1 hp. Now let's look at what happens to the Paladin.
With a 2k HP gap and 4% more damage taken, the pally would be sitting at ~2400 hp. Again, pally takes 4% more damage than the Warrior, so the incoming hit is 5200. Ardent Defender is active, so 70% of 5200 = Paladin takes 3640 damage and dies. Effective HP gap: 1240
Let's try a 6k hit. Warrior has 6001 HP, lives. Paladin has 3440 HP, is faced with a 6240 pre-AD hit, 70% of that = Paladin takes 4368 damage and dies. Effective HP gap: 928.
Wow, so as the hit gets bigger, the gap actually shrinks rather quickly. It's cut nearly in half by the time you get to the AD threshold. Obviously as soon as the hit gets any bigger it's going to break the AD threshold and leapfrog, but if your tank is taking hits bigger than 6k he shouldn't have time to be hit a 3rd time before getting healed.