So after some thought, I definitely settled on making him more of a spirit because that preserves more of what I've come to love about the character. The "feel and forgive the killer" resolution is extremely touching, both voice actors really knock it out of the park there -- especially in stark contrast to Cole's hurt and rage just moments earlier.
I think my initial feeling of "he's lost something either way" is deliberate because it's the point -- he isn't quite the same being afterwards, but the old self was hurt, so while there's some wistfulness (especially from the companion we don't side with) to the change, it's a good and necessary change no matter the direction. The following conversation and the Val Royeaux cutscene show his gratitude and happiness, and that is all that matters.
@Basement Cat: I think that is part of what his more-a-spirit-self refers to when he speaks of "washing clean" and the more-human-self calls "sticking" -- he affects others but they affect him in turn, too. Even when he encourages a change for the better in someone or gives them the easiest possible death, there's a LOT of pain in the world, and he hears and feels and shares so much that he probably needs a self-defense mechanism to bear it all. Otherwise he'd be sliding back towards despair (that word on his "gravestone" foreshadows his quest perfectly). More-spirit Cole has that in being able to make himself forget, more-human Cole needs to learn it (which will likely a more mortal version of forgetting) though he may also need it less since he says he has more of a choice of what he hears. As for the extra burden of having caused distress -- I think it'd hit all three versions of him hard, but thankfully both the more-human and the more-spirit Cole have better odds against letting it happen. The former will learn to read and understand more in the way mortals do, the latter's perceptiveness and empathy are SO through the roof that he is more confident about finding the right approach.