Good question! You’re absolutely right that the health bonus makes up somewhat for the missing defense. The way the damage equation works, though, defense is just much more important for a tank than health. The exponential in the damage equation magnifies the effects of changing defense on damage.
Guin’s “tank” stats are:
778 defense 1347 Health
So, converting the % boosts into numbers and putting everything together:
Wizard: +113 defense, +180 health
Fighter/Sorcerer: +149 defense, +112 health
Paladin: +157 defense, +112 health
This gives us:
Wizard: -20% damage taken, +13% health
Fighter/Sorcerer: -27% damage taken, +8% health
Paladin: -28% damage taken, +8% health
The increase in health for Wizard over Fighter/Sorcerer/Paladin just doesn’t offset the decrease in damage taken that the other classes would have, so Guin will die faster as a WIzard than as a Fighter/Sorcerer/Paladin.