One of the companies I used HipChat at before they switched to Slack and the only reason they ended up switching was there was a 4 week period where HipChat kept going down so one of the engineers setup a free slack room. It got used often in lieu of HipChat as it never went down.

I'd agree that they're both good, but if it's the main communication tool for your company the #1 priority is that it stays online.