+40 armor is 50% less damage. So, +10 armor is 1/4 of 50%. That comes out to 12.5% less damage, and 12.5% more survivability.
If you have 530 hp (a superior rune of vigor and nothing else affecting your health), then +15 hp gives you 1/17.66 more survivability, which comes out to something like 5.88% more survivability.
Now, +hp has the benefit of protecting you from armor-ignoring damage (which +armor does not). However, +armor indirectly increases the effect of healing (less damage taken means less damage has to be healed).
So, in short, +armor is more powerful, so centurion's it's better unless you really want extra protection from armor-ignoring damage.
I had a similiar dilemna when deciding between a defense grip or health grip for my scythe.