In addition to the rest of the comments, HipChat doesn't handle file uploads nearly as well, the message formatting is a joke, message history randomly gets era

In addition to the rest of the comments, HipChat doesn't handle file uploads nearly as well, the message formatting is a joke, message history randomly gets erased, simple things like posting a YouTube link in Slack will embed the actual video, you can edit message in Slack (HipChat has that silly "s/word/replace" feature). I use both on a daily basis and Slack is just better in every way.