Defiance: Tank Stance, 25% health increase *in addition to a 20% increase in healing*.
Grit: Tank stance, 20% damage reduction.
I 100% agree. The 20% increased healing along with 25% bonus health results in essentially similar 'toughness'. I decline to use the word mitigation b.c. Defiance does not reduce the amount of damage you take, expect for the minor Parry increase.

Take the following example:

10,000 Damage HIT after Defense
10,000 Base HP
Ignore Parry

Dark Knight:
-10% Debuff @ 67% Uptime = 6.7% Reduced Damage
10000 * (0,933) * (0.80) = 7464
10000 - 7464 = 2536 HP Remaining
Healing Required: 7464 HP

Warrior:
-10% Debuff @ 100% uptime
10000 * (0.9) * 1 = 9000
10000 *125% - 9000 = 3500 HP
Healing Required: 9,000 / 120% = 7500 HP

The two classes require almost the same amount of healing to offset the same hit. The warrior's mitigation is lower but the increased healing and hp offset a large portion of it.