What Ziggums said. One is offensive the other is defensive.
The armor tier is fine. DoG can 1-2 shot you in either armor because his boss fight is not about tanking hits, its a "final test" of your knowledge on how to dodge , learn his AI pattern [which the devs have repeatably stated in the past to others calling for a nerf to DoG] and how to prepare a proper arena for such a challenging boss.
The armor is the same tier, and its a matter of personal choice. Do you want Tarragon for defensive stats especially when grinding for materials or items or doing whatever boss in the world [not just DoG] or do you want offensive stats for faster clearing. Regardless of your choice, DoG is going to 1 or 2 shot you, no matter what choice you pick. That does not make the armor tiers "messed up". His damage is through the roof for a reason, you aren't suppose to survive multiple hits from him.