I think both paths are good and bad, in different ways.
On the human path, he learns how to help people by understanding their emotions. It's more difficult for him, but it also helps him to understand how his words/actions affect other people. It also helps them, because they know that someone cares enough to help them, and means that they can grow as people, overcome their problems, etc. I like that there is so much potential for him on this path, and love hearing his thoughts on how he wiped his own memories to protect himself from pain. On the other hand, I feel like I've robbed him of the benefits/abilities of being a spirit, perhaps stopped him from being able to return to the Fade if he wished, and generally put him on a path that could cause a lot of struggle and pain. The fact he can happily go to a restaurant with you and apparently not notice all the 'hurts' around him makes me feel like he's lost that aspect of his spirit nature, even if the kindness remains. Plus, I always feel like it suggests you don't really accept him as a spirit, you can only accept him if he tries harder to be human.
On the spirit path, he can help so many more people because he becomes more perceptive of their troubles, and because of him erasing his own memories, he doesn't have to carry the pain of any of it with him. He's able to finally let go of what happened with the original Cole, and on the whole seems a lot happier. During the outing to Val Royeaux, he seems happier than I've ever seen him, and part of that is knowing that you've accepted him for who he is - something he laments that even Rhys couldn't do. He's true to his original nature. The drawbacks to it are that if he wipes his own memories, he can never learn from his own experiences, meaning that something like his situation with the original Cole could happen again. I always fear that if he stays outside the Fade on this path, he might suffer the same fate as Justice. Although he can help more people, his help might not strike them as deeply as it might if they could remember.
I really like both paths, and I'm happy to do either depending on the personality/motivations of my Inquisitor. But, there is one thing I'm not as keen on with the spirit path - the last conversation in Skyhold. If Cole's more human, he stresses about the original Cole, the Inquisitor comforts him and he's appreciative of it. That's nicely supported by the way they end up sitting together on the wall. If he's more spirit, however, he wipes his memories of the original Cole, and when the Inquisitor asks if he's okay, he just replies in this deadpan voice something like 'You can keep using that name if you like, I'll stay here and help'. It really freaked me out, how odd he sounded. And the Inquisitor just... walks off. If there was no more dialogue, then they could've just ended the scene on that odd line. But no, he's stood facing away from the Inquisitor, and they just walk off. It just seems a bit... cold in comparison to the human ending to the same conversation.