I’m still thinking about this post and though I still don’t have a solid theory about who would win, I do have Thoughts that I just need to air out.

Emet SelchandSolas are both incredibly ruthless characters, but in different ways:

• Solas intended to “end/recreate” the world in a manner that avoided as much suffering as possible, bc despite initially seeing the people around him as weaker/lesser, he still cared enough to not want them to suffer
• This becomes even more important to him when his plans are foiled by corypheus and the quizzy mucking things up, and corypheus begins causing, well, a LOT of suffering. Through his journey with the inquisition, particularly if you stay on good terms with him, Solas will come to value people, despite his earlier judgement
• By contrast, Emet Selch SEEKS to cause suffering. As much as possible. He does not consider the people of the Source and its shards to be real people, and he determines that killing these people is therefore not murder. He is merely ushering their souls to a final culmination that will restore them to their REAL selves. To Emet, the suffering is necessary, and ultimately a favor to the very people he’s harming
• Solas’s ruthlessness is more directly tied to YOU, the player. If you befriend him, or take it a step further and romance him, he is more open to viewing the residents of the world as real people whose lives are valuable. But even THEN, it is not enough to deter him from his plans to remove the Veil and potentially end those lives entirely.
• If you are antagonistic towards Solas, it goes a long way to fueling a vicious streak in him that you only see hints of otherwise. He will, in fact, take pleasure in your pain and suffering. He will even remove your magically damaged arm with a clear sense of brutality.
• Emet Selch is canonically endeared to you as the player. Your character has an established backstory with him and you find that he, in fact, deeply cares for you. It’s what causes him to hesitate to kill you upon seeing you on the First shard, and leads him to instead extend a temporary truce. But in the end, even knowing who you were in his world isn’t enough to stop him from opposing you. Unlike Solas, Emet Selch has killed billions of innocent people over the course of millennia, and feels he’s come too far to let even a beloved friend/LI (depending on your headcanon) stop him.
• Despite choosing to fight you, it can be argued that Emet either consciously or subconsciously self-sabotages in order to end his own suffering and grant you victory. After his defeat, he smiles in death, and you later learn that he wanted you to live. Though you can’t avoid fighting him, it is established canon that your relationship with Emet Selch is complex and rooted in love (again, platonic or otherwise depending on your own perception)
• Solas is presented as a character that is difficult to persuade away from his chosen path, even when he consciously seems to want to do so. He is an immovable rock with a mission, and all the love in this new world he’s awoken to doesn’t seem to be enough to sway him.
• Emet Selch is also incredibly stubborn, but it’s made adorably clear that with enough prodding, he can be incredibly kind and helpful, especially to the people who mean the most to him. It’s also later learned that certain circumstances played a significant role in the ruthlessness he exemplified for so many years, and had those circumstances been different, he would have likely changed his chosen course, or not embarked on it in the first place
• Solas has been set up as a potential main antagonist for DA4; if this is indeed the way the story goes, we will likely learn a lot more about the full spectrum of his abilities.
• As it stands right now, Emet Selch has the advantage in both experience and raw magical power, but may actually lack a degree of viciousness that Solas does not.