It is a bit of a confusing message, but I think in their case these two were chosen based on whether or not they managed to be brave and try to make the most of their lives or, rather, whether they had just spent it disengaged from others, avoiding risks and any real discomfort.
I think angel dude was chosen to be an "angel" because even though he he coudn't be with his one and only, he still didn't give up on love and "bravely" took a risk by trying to date other people, and even when that failed to make him happy, he didn't mistreat his girlfriends and tried to be as "kind" to them as possible (in those circumstances). Whereas demon-guy was always dettached from people and aloof and didn't really allow anyone to get close, as he was even distrusting and resenting the seme even while having sex, before they died.
I can't help but think that maybe the reason that certain people who we the readers would consider angels became demons is actually just a metaphor for how society labels these people and not how they should be judged.