To answer your question: If you want to use dual monitors - do either of the monitors have built in speakers? If they do, do you plan on using them? If yes, HDMI makes it so you don't need an additional audio cable (in at least one of the monitors). If not, use Display Port for both.
The difference is that the HDMI 2.1 cable can output video AND audio signal, while Display Port has a larger bandwidth, due in large part to it ONLY outputting video (and not audio) signal.
For example, I have two monitors. One monitor has a built in speaker (it's also a higher resolution and refresh rate, 1400p/165Hz) and the other is a 1080p with no speaker. I use a Display Port cable to hook up to the 1440p/165Hz monitor to enable it to reach its maximum potential (refresh rate and resolution) and a separate audio cable to connect the audio output.
I use an HDMI cable to go to my 1080p monitor because it doesn't need an audio output option since it has no speaker and has low resolution.
Newer certifications like HDMI 2.1 (updated from 2.0) and Display Port 1.4a (updated from 1.4) on the video card ports makes it so you can use more of the HDMI and Display Port cable bandwidth.
I kind of like to think of it as... a big pipeline. On one end, your graphics card, it has a large capacity for rendering stuff, but it can't push it all through the pipe at once. It is constrained by the diameter of the pipe. The updates to the ports make it so more can go through the pipe.
Hope that helps (or if smarter folks can explain it better, I defer to their expertise!)