So your comparing
Grants a (x) point Damage Shield for (y) seconds
vs
Deals (x) Magic Damage and recovers (y) Health
Most V10 tanks I know are going for
Decrease Health - Deals (x) unresistable damage
At V10 purple, this is 35 extra damage that can not be resisted.
But comparing just the two you said
ABSORB HEALTH > HARDENING
as its damages them and heals you and anything that makes the target die faster, is better.