I consider HoK to be the most valuable stat for most builds because of how hard it is to get heals in this game. Whether you want HoK or more DPS, just ask yourself how often you were knocked out or have had to scale back your offense because your HP is low, and think if you would've avoided being knocked out or how much more HP you think you could've had if you had higher HoK. 12% HoK is pretty decent for AW and Keeper because both of them rely more on barrier, and should be enough to keep you safe for those times where you lose HP. It may be a bit hard to make use of the HoK on the keeper with a purely support build or if teammates are eating up all the kills. On the other hand, magic will just increase your damage by a few %, whereas going from 10% HoK to 20% HoK doubles how much you heal yourself per kill, which can be a lifesaver and indirectly result in you playing more offensively and doing more DPS than through pure DPS increase through magic.
My personal rule of thumb is to just stack all HoK items I have on all characters, except possibly 1 ring if it increases the damage or duration of a key ability. I have never been a big fan of glass cannon builds in most games, because after factoring in the reduced DPS and other consequences of getting knocked out/killed or having to reduce your offense due to low HP, it seldom made sense to go glass cannon. Some games are exceptions because they have optimal builds that are glass cannon, but imo in this game healing is king, and the enemies are too clunky and unpredictable for it to be worth it to choose a few % more DPS over the same few % of extra healing.