2x HiDPI is only 1080p and the UI feels a bit too large.
Let's clarify that "2x HiDPI" on a "4k" UHD display is full 3840x2160. True, the UI is a bit large but (unless you are using ancient pre-retina software) The "looks like 1920x1080" only refers to the physical size of th UI - it is rendered at full 4k resolution. You lose a bit of "real estate" because of the large UI - the impact depends on what software you're using, whether you're using it full screen etc. Most applications will let you zoom out, or reduce the font size, so you can still fit a lot of content at full 4k sharpness.
You will still get significantly more detail than on a real 1080p, or even 1440p display - and it is still rendered to single-pixel accuracy.
Likewise "looks like 2560x1440" gives you the same physical UI size you'd get on a 27" iMac, and displays
significantly more detail than you would see on an actual 1440p display. It is effectively a
5k image downsampled to 3840x2160. YMMV, but for my money you only notice the difference if you do a side-by-side comparison with a true 5k display, or go hunting for artefacts by getting nose prints on the screen while looking at 1x1 checkerboards or single-pixel scrolling lines. The YouTube videos on the subject use
simulated and/or magnified images to illustrate the effects (the
good articles are up-front about that).
If you spend the majority of your screen-time tweaking single pixels (without zooming) - or if you're running 3D software or games on a lower-end M series that can't quite hack the render-at-5k-and-resample thing - then it may make sense to stick with a 1440p display. However, only a 4k screen can display full-screen 4k content, and if you
occasionally need a pixel-accurate display it takes seconds to switch to 2x mode. There's also the issue that text quality on non-retina screens took a hit when Apple removed sub-pixel anti-aliasing. Unless you are
really on a budget or have one of the aforementioned specific needs, I really wouldn't recommend getting a 1440p or lower display today.
Yes, 4k is a compromise c.f. 5k - but the price difference is huge and the choice limited.