I've used both (though more HipChat). I find that Slack's UI is confusing for me. The relationships between channels and people and chats isn't as obvious as HipChat. And the UI feels "web-based" no matter what client I'm on. HipChat has nicer native clients.
The main advantages I've seen for Slack are:
- Support for switching easily between multiple accounts, if you work with different companies who use it
- Better support for using it as a document repository, searching through attachments, managing screenshots, etc.
Lots of people rave about how many features slack has, which I suppose is good if "more features wins". But it depends a lot on whether it's your central collaboration suite, or just something you need for chat features (HipChat has added video chat too recently).