Am I allowed to revive a thread this old? Hope so... came late to DA:I and just left the game at this point. Tried both ways up to the cutscene, but didn't made a final decision.
There isn't a right answer, it is more of what you think is better for him.
Letting him become a spirit again is to chose his happiness. In his scene at Val Royeaux it is obvious how happy he is now that he lost his human side. Without the human in him, he is back at his pure Compassion self, molded solely by the will to help others and finding fulfillment in this. There in no inner conflict, there is purity and, in it, peace. Not being a human, he doesn't share the pain of others anymore when accessing them, he can help them without suffering at the same time. The problem is this means he lost his ability to empathize, he can identify the problem and correct it, but all in a very detached way, he is not connecting with others anymore. He becomes complete, happy, but similar to a robot that is controled by one single directive - although with a sense of fulfillment.
That aside, chosing this path, as Cole states, makes him glad for seeing the Inquisitor accepted him for what he truly was without trying to change that (meaning that he saw his human side as something alien, not a true part of him).
Making him become a human is chosing his development. At Val Royeaux he is nowhere as happy as in the above, it is a bittersweet scene because he is not a simple spirit moved by one only feeling, but a complex creature with a whole plethora of emotions he is only learning about. And even once he gets used to it, he will never be as happy as if he remained a spirit, because that's what being a living creature is. You are filled with conflicts, the beauty of it comes from this complexity and everything it entails and not the purity you find in spirits. He is able to evolve, learn, form real bonds with others and connect to the world in a much richier way with a much deeper understanding of his surroundings.
In the talk, he admits he actually feared changes because he learned it meant losing what he already had, but is thankful for the oportunity to see changes for what they are - meaning that even though he didn't wanted to change, it was simply because he was afraid of it.
As I see it, making him a human allows him to evolve, but maybe that's only because I am judging it from a human's perspective. As Solas said to Varric, you shouldn't judge a spirit based on the concepts of what is good and right for a human.
This option, however, bothers me for two reasons: first, it takes away from Cole what makes him a unique character and makes him just another human, and, more importantly, I don't think being a human was ever his intention. It was simply a failed attempt to help the original Cole. He didn't became (half) a human because he longed for it, because he wanted to feel what was like, but simply because he thought this would help someone else... and it didn't. It was a misstep, an error, and he was stuck with it.
On the other hand, allowing him to become a pure spirit would be the same as killing all the emotions he acquired and would only develop much further if he became a complete human. Kieran says that being without magic is a sad thing, it's like missing a sense and you are unable to perceive an important part of the reality, living always in ignorance. Allowing Compassion to become Compassion again is similar to that, in my eyes; it is the same as handicapping Cole and taking away from him this new and empowering ability to elaborate the world from a much more complex perspective and truly connect to it (soft lobotomy?).
Really hard decision, but I think I will go with the spirit path because I believe judging what is better for him from a human's POV is inherently wrong. We should try to look at it from the perspective of a spirit and I think Solas is the best one to listen on this. Also, how happy was Cole in Val Royeaux as a spirit? If he can live forever in that state of complete extasis, I can't be sad for him.
Also, I think he can always revert it and go the human way again if he really wants it, the opposite... not so much.