In my experience, if a tool provides any business value at all, worrying about $6/employee is a complete waste of time. If Slack saves each employee 6 minutes per month then it has paid for itself.
Developer tools generally charge far too little for their offerings - price point is probably not even a consideration for most companies, as long as its within an acceptable band.
I use both on a regular basis, because our company has split personalities about which chat client is best.
I don't think it's meaningfully different or better than HipChat. The most obvious differences are that it automatically retries sending messages if your network glitches, and that it's proponents are more irrationally positive than HipChat's.
They're interchangeable. People who argue otherwise don't have enough actual work to do.
But my point is that doing that analysis is pointless, because the dollar amount is so low. The difference in price will never be more than a rounding error in a company's expenses, so it's not worth worrying about.
If someone at the company wants it, price point shouldn't be what's keeping the company from using it.
Developer tools generally charge far too little for their offerings - price point is probably not even a consideration for most companies, as long as its within an acceptable band.