Regarding size and resolution:
27" is better with 1440p. If you want 4k, it is better to increase the size e.g. 32".
Windows icons will be hell small on 27" 4k.
Recognizing the difference of having higher resolution (detail level, sharpness) has something to do with resolution, size and viewing distance.
If you sit around 3 feet away (only example) from the 27" monitor, there is little to no difference between 4k and 1440p.
Regarding resolution:
More resolution is better. More pixels means more working space (editing, etc.).
Better pixel density, sharper pictures or more details.
For gaming:
More pixels means more powerful GPU is needed, which means more investment.
For 4k needs serious investments. No more mainstream GPU for you in the next few years, only the enthusiast GPUs.
Scaling:
Since there are still tons of 1080p videos, etc. 4k is better than 1440p from upscaling point of view by far. 1080p videos look quite a lot better on 4k than on 1440p.
Current monitors technology:
Only 1440p can go 144Hz/165Hz (e.g. XB270HU, XB271HU, PG279Q), the 4k still can only 60Hz.
Monitor refresh rate is important for hardcore gamers. I have XB270HU.
If someone ask me which one I prefer for 27, I would answer, that I prefer 1440p. Go for bigger monitors for 4k.