What is the difference between DisplayPort and HDMI?

DisplayPort and HDMI offer similar benefits depending on the version, but DisplayPort is typically preferable for PC monitors. VRR, like FreeSync and G-Sync, is better supported by DisplayPort; HDMI currently only supports FreeSync. HDMI has wider compatibility with home audio/video devices, while DisplayPort primarily supports monitors and PCs.

What is the difference between HDMI 2.1 & DisplayPort 2?

DisplayPort 2.0 has also been announced, and it has an even higher maximum bandwidth than HDMI 2.1 at nearly triple the bandwidth of DisplayPort 1.4. This would theoretically make it possible to achieve resolutions of up to 16k with compression or lower resolutions with much higher refresh rates than is currently possible.

Should I use HDMI or DisplayPort for HDR?

If you have the choice between DisplayPort 1.4 (or 1.4a) and HDMI 2.0, DisplayPort would be the better option. In other cases, if a monitor only gives you the choice between, say, HDMI 2.0 and DisplayPort 1.2, HDMI could be the way to go for the HDR support, as long as all your devices support the HDMI version in question.