Tanking aura's

Heya..this is my first tanking char and ive read up on tanking with the various tank classes...

Im not so sure which is better in an instance...ive been tanking with devotion aura(damage mitigation)but ive' read that ret aura is good for tanking too.This i can understand..ret aura does holy damage and join that up with righteous fury,equals more threat

but devotion aura gives me more mitigation(more survivability)

Can an experienced pally tank give me some insight?

Thank ya kindly :)