TLDR Summary: Bulwark (Defense) and Life (Health) runes are fairly well balanced, but Bulwark, for most heroes will be slightly more beneficial than Life in mitigating damage from direct attacks. However, rather than stacking all Bulwark or all Life, usually a combination of both provides the best benefit, again, depending on the hero. When determining whether Bulwark or Life is better, check to see which will increase the stat they affect by a greater percentage. Aegis (Damage Reduction) is generally less beneficial than both Bulwark and Life in terms of reducing direct damage from attacks, except at extremely high existing defense and life levels. Damage Reduction, however, reduces “damage over time” damage such as poison and burn, where Defense does not.
In a previous write up, I compared Damage Reduction, Dodge and Resist. Resist is the least beneficial in terms of mitigating direct damage and either Damage Reduction or Dodge, depending on the circumstances, could be better than the other. For purposes of this post, I’m going to compare Defense, Life and Damage Reduction. Dodge is very circumstance specific, so I won’t address again in this post.
For purposes of my comparison, I used the stats on Greater, 3 Star, Rare runes – where the runes had the same attribute on both the primary and one of the secondary stats. For Bulwark runes, on average, this type of rune will give you 278 defense (185 primary and 93 secondary). Life runes of the same type give 627 health (414 primary and 213 secondary). Aegis runes of the same type give 6.69% damage reduction (4.54% primary and 2.15% secondary).
Damage received by a hero is calculated as follows: base damage of attacker * attack rating of attacker / defense rating of defender. There are other modifiers such as armor adjustment, magical resistance, elemental advantage/disadvantage and others. However, my comparison will just be using the generic formula above and you can adjust, as necessary, for any other hero or situation specific modifiers.
The following chart shows a comparison for two heroes (Icebloom and Yokozuna), showing the extremes of low and high defense. I’m using the defense amount of 278 on Bulwark runes as a baseline and then showing what the Life rune and Aegis rune would need to be to give equivalent benefit. I used Black Diamond's attack rating at around level 63, but the attack rating doesn't really matter in comparing value of these runes - changing it will cause total damage to kill to go up or down, but it does so in relative amounts so doesn't impact which runes are better. In the case of Icebloom, because she has very low base defense she benefits more from added defense. It would take a Life rune of 741 health or an Aegis rune of 15.12% damage reduction to give the same benefit as the Bulwark rune. Since at the Greater, 3 Star, Rare rune level, Life runes only provide around 627 health and Aegis runes only 6.69% damage reduction, the Bulwark rune is better. Note that the “Damage required to kill hero” assumes no healing taking place. Healing complicates matters and is discussed at the end of this post.
For Yokozuna, I used the assumption that he already had three very high level Bulwark runes equipped, which boosted his defense to 3,880. So the question is whether using the fourth rune slot for a Bulwark, Life or Aegis is better. Because of his extremely high defense level, you can see that the Life rune is actually better to place in the fourth slot than more defense. It would only take a Life rune of 454 health to be equivalent to the extra 278 defense, and since Life runes of that type provide 627 health, the Life rune provides more benefit. An Aegis rune, in this case, would provide the exact same benefit as a Bulwark rune because Aegis runes of that type give 6.69% damage reduction, which is what it would require to be equivalent to the Bulwark rune benefit. I didn’t do a third scenario, but if you did one where base health and defense were both high enough, than the Aegis rune would be better than both the Bulwark and Life runes. This would require defense of about 3,880 and health of about 8,800.
This can get fairly complicated, but an easy way to see which is a better upgrade is to calculate the percentage that a rune will increase the stat of interest and compare that percentage to what the other rune type will provide. In the chart above, for Icebloom, her base defense is 1,561 and base life is 4,158. A Bulwark rune will provide 278 defense, so that is 17.8% of base defense. A Life rune will provide 627 health, so that is 15.1% of base health. So between those two, adding more defense is better. Damage reduction is a bit trickier. To compare a Bulwark to an Aegis rune, take the percentage benefit from Bulwark and plug into this formula: 1 – (1/(1+defense percentage increase)). Then compare that result to the damage reduction on the Aegis rune. So in the Icebloom example it would be : 1-(1/(1+.178)) or 15.1%. Since Aegis runes of that type only provide 6.69% damage reduction, Bulwark would be better than Aegis. If you wanted to compare Life to Aegis, you’d do the same thing, except substitute “health percentage increase” into the formula instead of “defense percentage increase.”
So those are the basics. It gets more complicated when dealing with heroes with individual and team passive buffs. The undead are a good example. Zomm gets a 25% boost to health for being a slow hero and also gets a 25% boost to health because he is Zomm. The boosts are applied after including the rune benefits, so if you were comparing the benefits of Life runes to Bulwark runes, you’d have to adjust for this effect. In this case, Life runes have added benefit and in many cases this will shift the equation in their favor. However, Zomm is often run on a team with Yorick, so Yorick would give Zomm a 30% defense boost. In that case it might swing things a bit in favor of Bulwark runes (although I can’t confirm whether the 30% team defense boost is applied before or after rune benefits, like they are for individual passives).
Another factor is that even though Aegis runes are generally inferior to Bulwark and Life runes, except at very high defense and health levels, damage reduction has the benefit of reducing damage related to effects like burning and poison, where defense provides no benefit. I did some testing of this and proved it to be true. So, it is hard to quantify because it is situational, at least in comparing Bulwark to Aegis runes, it is relevant.
One more complication is when healing starts to be involved. All the above computations assume no healing happening. Once you start introducing healing, things get more complicated. The previous discussion will still generally apply, but there are concepts such as “over healing” and others that need to be factored in. Since this post is already pretty long, maybe good to discuss in another post sometime.
As always, happy to hear different opinions. This topic is fairly complex, so I wouldn’t be surprised if I’m not thinking about all the factors involved.