Don't forget about life steal! It's pretty interesting as well and you don't see it on a huge amount of items. Total weapon damage, and attack speed all affect it, so it's a pretty complicated way to go about getting health back.
Against erebus or QoT it's absolutely critical that you reach an armor level capable of negating their armor reducers.
It follows this equation: 100*(1-(2500/(2500 +A))), where A=armor.
Here's a very, very simple google doc that'll let you put in your armor value to get your mitigation, which is already covered on demigoddb to an extent, but this'll let you figure what it would look like at negative values and unusual positive ones you reach when you're affected by bite/spikes.
One thing you'll see is that damage scales incredibly quickly at low and negative armor values.
Really when it comes to total health there's a few things you've gotta ask yourself:
- Do I have monks/priests?
- Am I engaging in extended fights or being worn down by snipes/spits/RoI/etc.?
Ultimately I would say that the way items are built debate over what you should prioritize is a bit academic. Personally if I could pick 15 health per second or 600 total health I would pick health per second in many cases, such as when regulus is sniping me repeatedly to wear me down or when UB is spitting and running over and over again.
It's all a moot point though because there is no regen focused item out there before artifacts, just as there really is no pure health or pure armor item after scaled that is really worth its cost. The items everyone picks are hybrid items, and not even because they appreciate every aspect of the item, but because the pricing is set to values which guide your hand regardless of your appreciation for the item as a whole.
Banded Armor, Nimoth's Chest, etc. are items that you're not going to pass up on no matter what, even if you're unconcered with the regen on banded or armor on nimoth's, there's just a dearth of alternatives. Same idea with Narmoth's Ring. I'm sure many people buy it because of the health/life steal and wouldn't care one bit if in the next patch it had 600 armor instead of 20 health per second.
Personally though I absolutely love regen. Even 30 health a second is pretty major, there's really a lot more downtime than people realize between fights, not to mention fights themselves can last quite a while in some situations.
I would even go so far as to say that I'd be interested in regen potions, maybe they could either be cheaper than normal potions or heal for more, but do it over 10-20 seconds (a heal over time skill would be a cool ability for future DGs too).
Really though I would say that priests and monk idols have such a dramatic effect on virtually every battle that in the end that health stacking is virtually always your best option once a basic level of armor is obtained. If those units gave a static amount of health back instead of a percentage you'd probably see way more diversity out there. You could make priests scale off of unit health or damage upgrades and monks already have four ranks which could be tuned to whatever you felt were appropriate.