I faced this dilemma before eventually stripping Vela in order to emblem up Noor, because I use Noor in 15ish raids daily and virutally on all PVE non-farming levels.
There were arguments against doing this (some valid) but I ultimately went with the enjoyment factor. However another valid aspect of it is that the stronger you make your most used heroes the more daily loot you will gain from raid chests, a subset of raid tournies, challenge events, titans (if you use the hero). This leads to more heroes and more ascension mats and more emblems which feeds back into increasing your overall strength, including your war bench and war defense. So by making your dailies stronger you make every other aspect of your game stronger
To add to this - Lady Loki is close to being the best hero you can have at the moment with so many DOT and ailment inflicing heroes in high diamond. I have been embleming mine on the attack path but half way through I may switch to defense. She is already very bulky but she is such a critical hero that increasing her survivability may result in some matches being able to be turned around (especially if you have minion or heal support heroes to bring her back from the brink). Attack will make her special stronger obviously, and although the bulk of her damage comes from DOT reversals, the attack boost to the initial damage can still be noticable. I really don’t think there is a way to go wrong here.