I'm Getting More Confused about a WAR's Ability to Tank

[Rbstr]

I don't see a feasible logic for the 25% healing/HP vs a flat 20% mitigation especially since raids always have some kind of tank cruncher.

Well you should see feasible logic these two things things are nearly identical in practice (it's only a 20% healing buff, though):
Two tanks have 100hp as a base value. They take an attack that does a base damage of 100hp.
A paly takes 80 damage (20% reduction) and is left with 20% of his HP.
A war takes 100 damage and is left with 20% left over (since he has 125 to start)

Say the healer uses an 80hp heal
Paly gets 80% of his HP back
War gets 76.8% of his HP back (heal hits him for 96 HP)
This is hardly any different in the end with typical over-healing. Lustrate has the exact same effectiveness between the two jobs.

The real "difference" comes in the additional mitigation options that really matter. Sentinel, Rampart, HG (and maybe bulwark) vs. Vengeance, Inner Beast, Holmgang.
Either set can be better depending on the timing of the fight. Inner Beasting every tank-buster is often better than what a Paladin is capable of.