If you’re tanking Plantera though, you’re probably using exactly one weapon: Fetid Baghnakhs. In which case, you’d get a hell of a lot of damage from Titanium Armor which would not only contribute much more than Thorns (especially because you’re probably using a cross necklace), but do so by big enough a margin to more than make up for the defense difference. Though in Adamantite worlds you’d use Turtle.