I dont have a problem to downscale if its better and faster for my system.
You'll get the optimum image quality and lowest GPU loads in either "3840x2160" or so-called "1920x1080".
In "3840x2160" the UI size (menus, buttons, system fonts etc.) is a bit on the small side for most people (but you might be happy with it on a 32" screen). TL
NR: if you're already happily working in "3840x2160" and your eyeballs haven't exploded - you're sorted. In "1920x1080" they're double the size making them a bit
too big on a 32" display.
Note that (unless you jump through hoops to enable "low resolution" modes)
these are both full 4k 3840x2160 modes. "1920x1080" just doubles the size of UI elements it doesn't change the resolution (unless you're running ancient, pre-retina software). In "1920x1080" - if you hide the menu bar and dock, or work in full screen, and zoom your content to taste, it will depend upon the design of the app you are using whether the control palettes and buttons take up too much space.
The other modes are a bit more complex because they don't correspond to exactly 1x or 2x the actual resolution of the screen. What happens is that, effectively, everything gets rendered to an internal buffer that is 2x the linear resolution of the listed mode (i.e. "2560x1440" gets rendered internally at 5120x2880) and then scaled by the GPU to the actual 3840x2160 resolution of your screen. The end result is that the UI elements are the same physical size as they would be on a standard res display at (say) 2560x1440p - but there is still far more detail than you'd get on an actual 1440p screen.
The internal buffer and rescaling by the GPU is what causes the performance hit in those modes (...worry if you've got Intel UHD graphics, but a MBP 16 with a discrete GPU, or Apple Silicon, shouldn't break a sweat) and because it is being scaled by some fractional ratio you'll see some "artefacts" if you lean in and look at actual pixels. IMHO that's not an issue on a 27" screen in general use, typically viewed at arm's length - but if you have a larger screen (e.g. 32") and don't compensate by moving further away and/or you do a lot of pixel-accurate work, your mileage may vary.
Apple have made it somewhat misleading because of the way they label the modes as if they were actual resolutions (In the latest few MacOSs they've dropped the "looks like" qualifier) - and they've bundled the native, 2x and fractionally scaled options all under the "Scaled resolution" heading. Also, the Default/Best for Display setting seems to vary with Mac model, OS version, display model and the phase of the moon...
