Both of these got me thinking about the differences in the apps and for me they are both wrong. Both designs are good enough for my everyday communication. The only reason Slack is winning is because of the multiple account logins. At work, we used Hipchat, and it was a fine product that got the job done. Where it started lacking is when I would get invited to another companies account. The only way to use both was to have the browser open for one and the app for the primary. It was horrible.
That single feature alone made me champion the move to Slack. Of course, the ability to create public channels so open source projects, and teams can move off the dreadful IRC is another wonderful feature.