As far as I know, a heat pump and an AC unit are essentially identical. Although some heat pumps can be reversed, and act as heaters in the winter. But one isn't more efficient than the other, they employ the exact same physical refrigeration cycle.
Oh, I had them filed in by brain under “more efficient temperature control” but it looks like they don’t have an advantage over ACs when cooling. (Just over typical heaters, when heating). Oops.