That depends on the nature of the group I was in, I think. If I cared particularly about the regard of the other members of the group, and did not feel like I had a secure position of status in it, then I would probably prefer receiving the message by PM. If I felt secure in my social position, I would probably prefer that the message be posted openly, so that other people could appreciate it.

People are complicated. I've put a lot of effort into working out how to deal effectively with others in social situations (working my way up from a very low starting point I'm afraid,) and one of the lessons that has served me best is to not to suppose that even my best tried and tested heuristics will apply to everyone, and to be prepared to discard them when necessary.