I lean toward displayport by default, but that's mainly because I run a TV as a second monitor and most cards only have one HDMI slot.
Saves the need for an adapter, if you only run one screen and there isn't some specific limitation you might run into via HDMI it doesn't matter. Do check your monitors manual though, sometimes (albeit rarely) you run into something weird like freesync/gsync problems or lower refresh rates even with sub 4K screens.