To me, Cole already made the decision himself when he became flesh in the first place. I feel I’m respecting his decision by letting him continue on this path. It is a very difficult path - as Solas says at some point - but also very rewarding if he gets to understand humans better in order to help them in better ways too (although he’ll have to be more imaginative as he can’t make them forget or act unseen anymore).
I was about to type out exactly this! I think it's all about respect, and I feel like I'm respecting Cole's choices more by keeping him human. And now...I'll just kinda repeat some of what you said because I got it into my head that I was going to type it all out...
The choice is tricky, and there is no right answer because Cole was on a journey, but didn't get to complete the journey himself. He was going along happily--the way he chose to be--until he became worried because of all the binding stuff. He was happy enough in his state of in-between being that his solution to the problem was to stay as he was and be bound to someone he trusted. He didn't want to become more like a human or more like a spirit--we have to impose our decision on him, so neither answer can actually be right.
Also, as was already pointed out, Cole doesn't change his nature. He still goes about helping people--he just does it differently depending on which form you encourage him towards. It's a superficial choice because we aren't changing his actual nature of compassion, but if FEELS like a really important one because we're making it for him and because we think what you are should change what you are. It kind of goes along with Solas' "spirits are people" conversation--if compassion is compassion, then it shouldn't matter how much his body is a body.
As for my decision, Cole wasn't just a spirit before. He was unique and he was changing by force of his own will--I think that's where the idea that he wants to be human comes from (that's where I thought Varric got the idea from anyway). Cole made himself that way. It felt right to him, so the human choice seemed (to me) just as natural for him as the spirit choice would be. He went with the form of Cole because it was important to him. I didn't like how easily he shed something that had been important to him after becoming more spirit, and I felt like I was undoing his personal progress and choices when I went with the spirit choice and more like I was respecting his journey when I went with the human choice. Cole deciding Cole was unimportant could just as easily be seen as another step in his progress, though. I felt like it was progress I forced him to make by making him a spirit, but it could be argued he moved on from Cole on his own and that it is evidence that he would have chosen the spirit choice had he made the decision himself.
So it had to do with all of that for me and not any sort of "comfort" gained because he was "more like me" as a human. I think spirit Cole is awesome, and I understand why people feel like they're respecting his true nature more when going with that choice, but that's why I tend to go human.
Also, I think it is a great opportunity to disagree with Solas. It's one of the few cases where the expertise of Solas doesn't really come into play since it is all about Cole as an individual, so our opinions are just as valid as Solas'. It makes me feel bold and happy because you have to concede almost all other points to him.