After all the discussions about resolution, let's try and summarise this a bit:
5k (5120x2880) @ 27" or 6k @ 32" (220ppi/Retina) - the optimum resolution for MacOS at eye-watering prices and not a lot of choice. If that's out of your price bracket, don't loose sleep over 4k vs. 5k comparisons!
2k 2560x1440 (110ppi) - outdated - we used to think that it was the bee's knees when the 27" Cinema display and the first 27" iMac came out. These days, MacOS is increasingly designed for "Retina" displays though. If you're happy with it and haven't been ruined by using a 5k screen, fine. What it won't do is display all the detail in 4k content or get close to the resolution of modern digital cameras. With typical eyesight at normal distances you'll see pixels.
4k UHD (3840x2160) @ 27" or more (~160ppi/Nearly retina) - good compromise between the previous two. The screen modes/scaling thing is complicated and widely misunderstood/badly explained - but all the common screen modes
show more detail than an actual 2k display. It takes seconds to change screen modes to choose the best for a certain job. It can display 4k content with full detail and show more detail on photos than a 2k screen could.
"Macs have to run a 4k screen in 2k resolution" -
No. Apple uses confusing terms here. The standard modes offered by MacOS on a 4k or higher display
aren't resolutions but instead are various ways of scaling an "internal" 220ppi image to fit a 3840x2160 screen
. This is needed because MacOS only supports two fixed sizes for its user interface elements. All these modes display more detail than you'd be able to see on a 2k screen and you could use that to zoom out/reduce the font size in your applications to fit more content on the screen.
- "More Space/3840x2160" gives a pixel-perfect 4k display without scaling the user interface - but you'll probably need a 30"+ screen to use it comfortably.
- "Larger Text/1920x1080" also gives a pixel-perfect 4k display but the UI elements are doubled in size. Which, unfortunately, makes them a bit too large on a 27" screen, wasting space (still completely usable, though, and doesn't affect your ability to zoom out or use smaller fonts in most applications).
- "2560x1440" scales the screen to make the user interface elements the same physical size as on a 2k screen, but still with a lot more detail than you'd get at 2k. You effectively see a 5k image downscaled to 4k. However, because that's not an exact 1:1 or 2:1 scaling this means the result isn't quite "pixel perfect" & also takes a bit more GPU power. For most cases this is a perfectly good compromise that gets pretty close in clarity to a 5k display when seen from a normal viewing distance, if you don't go hunting for artefacts.
The GPU load isn't so much of an issue with current Apple Silicon GPUs - but if you're doing heavy graphics rendering of any sort at 4k or 5k, remember it's going to take 4x the GPU power & choose your Mac appropriately!
I wouldn't pay money for a new 2560x1440 display today unless you have a very specific use case.