But +18 Defense is constant, +36 HP requires healer to restore after damage is taken.
Mainly +18 Defense is helpful for attacks mitigated by defense stat.
While +36 HP helps against counterattack damage, and damage over time, both of which bypass defense stat.
See also
(General game questions for more experienced players - #25 by Kerridoc)
( Hero durability - statistics )
(🍚 Talents: defense points or health points? - #34 by yelnats_24)