I'll list some notes, maybe this will help you.
4K
- Productivity favours screen additional real-estate (pixels).
- However, scaling becomes a factor depending on your eyesight.
- If you run a 4K monitor but the content is too small for you (screen size vs. seat distance), you'll have to scale it back up.
- Great image quality (IQ).
- Less aliasing (jagged edges).
- More pixels (8 million) also requires more bandwidth / GPU power for intensive 3D applications, like gaming.
- Scaling is ideal for emulation.
- 60 Hz
- For casual gaming, it is the standard
- Pushing about 60 Hz will either require a very modern GPU or will only be applicable in less demanding titles.
- 4K are becoming more common and affordable.
1440p
- Considered the sweet spot between real-estate (pixels) and IQ.
- Common resolution for monitors today (more affordable)
- Good IQ.
- Anti-aliasing still required (TAA etc.)
- Less pixels than 4K (3 million vs. 8 million) but requires less bandwidth / GPU power.
- Poor scaling.
- Plugging in any console or running a emulator that does not render at 1440p will look fuzzy.
- 120 - 165Hz
- Great for any game that can support uncapped framerate.
- 120Hz 1440p is technically easier to run than 60Hz 4K.
- Work / productivity / browsing is also improved (smoother).
- 1440p / 144hz monitors are similarly priced or cheaper than 4K /60hz monitors.
Gsync/Freesync (VRR) can be applied to both resolutions.
Context / my experience:
I have my PC hooked up to a 4K tv (C9 OLED/120Hz/VRR) and a 1440p monitor (165Hz/freesync/IPS).
I prefer the monitor for day to day work and gaming that requires a mouse/keyboard. Any controller gaming... 4K TV.
No experience with ultra wide. No experience with a 4K
monitor.
I'm heavily biased towards high refresh rate / VRR wherever possible.
I've been using 120hz since Samsung 2233RZ. I can't go back to 60hz...
Recommendation will depend on what is more critical for you (Productivity, casual gaming, emulation etc.).