There's no one "win condition" to this, and that is part of what makes this character so great. All three "versions" of him -- the state we first meet him in, the more-spirit and the more-human route are recognizably, credibly the same person, and both solutions to his personal mission help him get better. It's really down to preference and to which aspect of him you're more drawn to.
That said, I prefer the spirit path. As I said before on other threads about our compassionate little friend, I am quite allergic to the human superiority complex, to stories that elevate humanity as the highest attainable prize, to our tendency to value the most what is most like us and disparage what is different as inferior. Going the spirit path is partly my attempt to deliberately defy that harmful "more like me = better" reflex. I also like the challenge involved in having to think harder and deeper to understand him, bond with him and make room for him so he can flourish in his own unique way. It can be difficult, sad even, but it feels more "fair" than to expect him to go the whole way for us while we return only a minimum of effort -- he already has changed, learned and and given so much, let me do my best to extend the same to him. Besides, it's an incredible opportunity to explore what relationships between mortals and spirits could be like without fear and prejudice. That is very well reflected in what he says in the "dinner" cutscene, in his joy and gratitude for believing in him and accepting him as he is.
Also, when Solas first talked about befriending spirits, early on in Haven, I thought it'd be awesome if I could experience something like that. Along comes Cole and grants my wish. 
Both Solas and Varric are undeniably biased. Both prefer to focus very strongly on one aspect of Cole while handwaving the other away. Still, Solas is the one with the greatest knowledge, the only one who truly knows both sides of the Veil, which is a definite point in his favor. Varric's approach makes me uncomfortable -- it works in its own way, and as I said I enjoy Cole's more-human development too, but at the same time it feels almost cruel to force "lessons in humanity" on a very dear, very kind friend in a moment of extreme personal trauma and distress. Pain is a terrible teacher, and one that compassion incarnate probably deserves the least of all people in the game. I'd feel different about changes born from Cole drifting more towards his human side on his own accord.
Finally, I'm simply ... awestruck and blown away and very deeply touched by both his spirit-path resolution scene and some of his spirit-path banters. Solas and Cole are pure poetry together, and the way you can see Cole's rage and hate crumble, fall away and transform into mercy is an absolute masterpiece of writing, voice acting and modelling/animation. In such a bleak, war-torn world with so much riding on my Inquisitor, it was just beautiful to see that such unfettered compassion was possible. In a way, my Inquisitor needed that as much as the people need the Maker-blessed hope she represents to them.