Nice article ! I instinctively went with DP whenever was possible (also thanks to Dell for being among the first adopters of DP on their laptops) since I felt that with higher bandwidth one could do more or the hardware will struggle less to do the same as with HDMI, without knowing the technical details as you clearly explain. You mentioned cable quality a couple of times and this is important and unfortunately most consumers ignore this and buy the cheapest available cables then they wonder why the image quality is lacking. Is there any practical way for consumers to compare or benchmark the capabilities of their cables ?
Because HDMI and DisplayPort are digital signals, cable quality has zero impact on the actual image quality. Either the signal gets to the destination and the cable works, or it doesn't. Degraded signals usually cause a full loss of image, sometimes periodically and sometimes they just never work. At least in my experience, I've never seen a digital signal 'look better' with one cable vs. another.
The biggest problem with lower quality cables is that they often can't handle higher bandwidths, or they have to be shorter. For example, I bought a (cheap) 3m DisplayPort cable at one point. It could do 1080p and 1440p at 60 Hz just fine. 4K at 60 Hz caused a flicker -- meaning, the display would black out and come back on every few seconds. 1080p and 1440p 144 Hz either did the same thing, or simply failed entirely and the graphics card drivers apparently detected this and removed the option after I tried it.
The same goes for any adapter, like mini-DP to full-size DP. They can cause a loss of signal integrity, which can then cause periodic display blanking.
Generally speaking, my experience is that HDMI and DP cables either work for the resolution you want to use, or they fail. Most 2m cables will do just fine up to a certain point, beyond which you need a higher quality cable. DP8K certified cables should work for just about any signal, and certified Ultra High Speed HDMI cables should also be good. If you want to step down on HDMI, you can also get a certified Premium High Speed HDMI cable and it should do up to 4K 60 Hz at least, possibly more.
Something else to note is that if you want to use a longer cable -- 3m DisplayPort or 5m HDMI, for example -- you'll definitely want to get a certified cable. The certification 'proves' that the cable has been tested for compliance. Non-certified long cables are very likely to fail at higher resolutions and/or refresh rates.