Who cares? I don’t. I bet if I asked everyone I had contact info for, only a few would prefer one or the other (though hopefully one in particular).
Why do sites like Flickr still offer both as an option? I consider myself more knowledgeable than most on the matter simply because I know what the two represent, let alone what the technical differences are. And that being so, it makes no sense to put both standards (if not different versions of each) in the auto-discovery tags, especially for big sites like Flickr and consumer blogging software like WordPress. Only a rare developer building an application might care, and even in that relatively rare case I could reasonably assume they would be willing to read some developer docs. For the rest of us, our feed readers could care less. So please stop confusing us.
This seems like a dying issue. And then I see it on sites like Flickr! Tisk tisk.