- Native 1440p (2560x1440) on 27" is perfectly fine. This is what all non-Retina 27" iMacs used.
- The best is obviously 5K on 27" (exact double the pixels of 1440p in width and height meaning 5120x2880). This is what the Retina 27" iMacs and the Studio Display use.
- 27" 4K in "looks like 1440p" uses a non-integer scaling (the UI is rendered in 5120x2880 but then downscaled to 3840x2160 to match the screens 4K resolution.
- This still looks smoother than a native 1440p screen but:
- using a.e. Photoshop may be limited if you depend on a 100% view as the non-linear scaling makes it impossible
- the additional scaling effort reduces performance depending on the application you need. This video may be interesting if you never heard of the problem:
- If you use a 4K screen of the same size as your old one and select "looks like 1080p" everything looks the same size as on your old one but you gain the smooth "Retina" experience and don't loose performance as this is an integer scaling of 2 (1920x1080 > 3840x2160).
- But this means everything is larger and you therefore have less "space" than with a "looks like 1440p" scaling
Here you find some more infos/discussions on scaled resolutions on 4K displays:
To sum up:
- If you can afford it, on 27" go 5K.
- 4K at "looks like 1440p" may look almost as good but GPU-dependent applications may suffer due to the additional scaling effort.
- 4K at "looks like 1440p" gives you a smoother, more detailed image than native 1440p (2560x1440).
- I have not seen it, but on a 32" 4K at "looks like 1440p" also seems to be ok looking. Native 1440p on 32" is too low-density and looks pixelated.