Wiki article on armor rating. The first thing you should note is that for armor effected damage +x armor will always reduce the damage received by the same percentage regardless of the starting AL for a given value of x.
+5 armor is an 8.3% damage reduction
Lets assume a level 20 character with a sup vigor and a sup attribute rune. So 480 + 50 - 75 = 455 hp
If you add a +30 hp mod you bring your hp to 485, and the +30 only makes up 6.2% of your total hp. If you increase your hp from other sources, this percentage decreases. For the +30 mod to be above 8.3% of your hp, you would need to be at 361hp with the mod (30/0.083).
But even then it only performs as well as the +AL if your either not being healed, or your only taking armor ignoring damage. For PvE I'm not aware of a single enemy that does armor ignoring damage that doesn't also use its basic attack. And the only armor ignoring basic attack I know of is if they illusionary weaponry up. So apart from the illusionary weaponry enemies, every PvE enemy will cause the +al mod to take effect.
On the other hand +hp will only work in two situations:
1 - You take the damage to pull you under 30hp with the mod and you get healed before dying. Since your at <30hp, this will only take a wand or two.
2 - You manage to do something useful before dying, like getting off a skill just before the fatal blow comes.
In all other situations it does nothing except let more hp get lost to saccing or let 3 more damage per hit slip through prot spirit.
For PvE if either of these cases come up, you got really lucky.