Warrior is a GCD tank, Dark Knight is a weave-heavy tank. Instead of just a GCD resource for Bloodspillers, there is an off-GCD resource for counterpart abilities like Edge of Shadow.

Warrior is more about self-heals while Dark Knight leans more on mitigation and absorbing damage.

Short cooldown tank abilities have gained some equivalence to The Blackest Night in Endwalker, by having a combination of barriers, raw mitigation, regens and self-heals applied to them, and this combination of effects might be proving a little better against the variety of attacks we get in this expansion such as dot-busters, but it was definitely one of the best tanks in this area in Shadowbringers.

This is how I see them as different. These are the differences I explain to new players. Some people want to play something faster-paced like Dark Knight and some people want it to be slower like Warrior.