TLDR Summary: Attack runes are by far better than Damage % runes in increasing overall damage output. However, Attack rating does not increase damage from effects such as Burn and Poison, where Damage % runes will. Attack runes also increase overall damage output more than Crit and Skill runes, but Crit has other benefits for certain heroes and in certain circumstances.
There are 4 rune stats that can increase damage output: Attack, Damage %, Crit and Skill. Skill runes don’t appear to be working properly at the moment, except that they do at least increase the Crit value slightly (but not as much as comparable Crit runes). For that reason, at least for now, Crit runes are better than Skill Runes.
Crit runes and Damage % runes increase damage by a fairly similar amount in the long run, as the relative % value you get from comparable runes are fairly similar. For example, Greater 3 Star Rare runes give you about 4.4% crit (as the primary stat) compared to 3.7% for damage runes. So over a long fight, crit is a slightly better stat for increasing direct damage. Crit rune damage will be more spikey, in that when you crit you get 2x or 2.5x damage, while the Damage rune will give you a more consistent % damage with each attack. If you are trying to knock out a hero in one hit, then Crit will be better potentially, in those circumstances. Also, certain heroes benefit more from Crit in that they might proc specials (like on Goblin teams, Indigo, etc.) and they might also get a higher multiplier (like Black Diamond).
That leaves Attack runes to talk about. I’m going to compare these to Damage % runes, since they serve similar purposes in that they consistently increase the damage output of each attack, but in different ways. In the first table below, I’m comparing the damage output of Bovus attacking Tsume (both level 65, six stars and fully ascended, but without runes) using a basic attack. In the first two columns I’m showing the damage output comparing Attack versus Damage %. I used the average Attack rating and Damage % provided by Greater 3 Star Rare runes, which was Attack of 214 and Damage of 3.7%. You can see that the damage output when adding Attack was 1,483 compared to 1,232 when adding Damage %. One reason is because Attack gets double benefit from being increased. Base damage amounts are calculated on Attack rating (49.5% of Attack in the case of a basic attack) and Attack rating also increases the damage multiplier (multiplier is Attack rating/defender’s defense rating). It seems that DB did not assign high enough values to Damage % runes to make them comparable to Attack runes.
In the second and third columns, I’m showing that it would actually take a Damage % rune of 25% to be equivalent to an attack rune of 214. Even Greater Legendary 5 Star runes only provide about 7.6% damage increase, so this just reinforces that they undervalued Damage % runes by a significant amount.
Damage % will outperform Attack at very high levels of Attack, so I’m showing in the fourth and fifth columns the level of Attack at which Damage % runes start being more effective than Attack runes. It would require an Attack rating of 11,670 before this would happen, so effectively it is out of play in the current environment.
I know the question of whether the defender’s defense rating makes a difference is bound to come up, so the second table below address that. Similar to how the attacker’s attack rating doesn’t matter when comparing the value of defense and life runes, the defender’s defense rating doesn’t matter when comparing the value of Attack and Damage runes. For comparison purposes, I left everything exactly the same as in the first table, except I used Alexandros equipped with 3 legendary bulwark runes to bring his base defense up to around 4,000. As you can see, the results are identical to the first table in terms of comparing the benefit of Attack and Damage runes. It still takes a Damage rune of 25% to be equivalent to an Attack rune of 214, and the Damage rune starts being better at the same 11,670 Attack rating.
As mentioned in the TLDR summary, Damage % runes will increase damage from effects such as burn and poison, where Attack runes don’t increase that damage. However, I don’t think that benefit is enough to outweigh the benefit of Attack runes, as “damage over time” effects are more situational.
The question of stacking Damage % may also come up. Unfortunately that doesn’t really help the situation and actually only makes it worse for Damage runes. The third table shows what happens when you stack three Greater 3 star Legendary Attack runes compared with three Greater 3 star Legendary Damage % runes, taking it to the cap of 30%. The result is that you do more than 3x the damage with stacking Attack compared with stacking Damage %. A lot of this has to do with the double benefit you get from Attack rating. Also, to achieve the same damage results as adding 1,935 in Attack rating, you’d have to add runes that would provide 324.2% increase damage (well beyond what is available and also well beyond the 30% cap).
Hopefully DB will recognize the issue and not only give Damage % runes a boost, but also consider doing the same for Damage Reduction and Resist runes, as they are also underpowered.
As always, if you have comments or suggestions, or find some error in my logic or math, I’m happy to hear about it.