Is this the good old discussion about mitigation vs HP increase on one hit on one hand, and mitigation vs HP increase on successive big hits?
Mitigation is king when it's about sustained damage that starts going over HP thresholds, this is why a ToB based tank will shine on sparse outbursts of incoming damage, but as soon as it drags on sustain, or goes over their thresholds, they're suddenly losing a lot of steam and require more healing to keep up.
This also doesn't exist in a vacuum since there is party healing and support that will also apply and that's why mitigation has been king as soon as SE tried increase outgoing party wide damage in Abyssos+ (notably with the party wide dots).