After careful consideration, my answer is:- Neither.
My Warden romanced Morrigan the friendship path (gain her approval, get the grimoire, wait for Morrigan to ask if there can be more between us and then bang her). Then, he recruited Loghain and made hardened Alistair co-rule with Anora.
When Morrigan proposed the Dark Ritual, he rejected it because Morrigan does not give any tangible reasons for preserving Urthemiel's soul. Is she going to study or conduct research with it ? What could an untainted Urthemiel's soul be used for exactly ?
This leads to the second reason to be against the Dark Ritual. Making Kieran shoulder the burden of housing the Old God soul without him having any say on it is no different from Dorian's parents wanting to force him to be someone he is not and his father was willing to use blood magic to make that happen. Recall that Morrigan and the Warden engage in a blood magic ritual to force their offspring to lead a life that he did not ask for. In many ways, it is no different from parents who treat their children as merely a tool to carry out their dashed hopes and wishes instead of a sentient being that should choose their own path someday.
That is probably the biggest reason of me being against the Dark Ritual. I was the firstborn of an Asian family, the son who was shouldered with all the wishes and hopes of his parents and as such, was never really allowed to choose a life path he wanted until I rebelled and we had massive arguments where I won. I do not want an innocent child to be shouldered with the burden of carrying the soul of a powerful ancient being without him having any choice in it, much in the same way I do not want anyone to be raised and driven to study solely to be a medical doctor that gets married, gets a big house and pops out the typical 2.5 kids and have hobbies that his parents had to approve and control, has to eat food that his parents had to approve and control, etc.
When the time comes, Loghain made the killing blow and the Warden survives to step into the Eluvian with Morrigan (who is surprisingly warm and receptive of him even though he did not do the Dark Ritual).
In Inquisition, you get a normal Kieran that is a very sweet and adorable kid instead of the autistic OGB Kieran. Furthermore, you end up with Stroud over Loghain who you can sacrifice at Adamant.