Slack the data is on their servers. I'm sure they do some stuff to try and ensure privacy, but afaik they don't have an option for user provided encryption key (for example) to make it so Slack can't even see the data. So they'd potentially be vulnerable to hacking or government eavesdropping.
Hipchat has the option of setting up your own server so you can secure it however one sees fit.
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Uncertainty is an uncomfortable position.
But certainty is an absurd one.
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