It depends on what your use is but also which specific monitors (inch sizes) you are comparing.
It is easy to suggest that the extra vertical pixels from a true 4K are useful and they are, I'd always choose an equal or larger horizontal inch screen 2160p, which also makes it taller, obviously, and possibly too tall if sitting only a couple feet away and it's 39" or more.
On the other hand if you are contrasting a 4K monitor that has a smaller horizontal physical measurement, it might seem as useful to have the wider screen from the 1440p and in gaming, if your games support it, it is easier on the video card to push fewer pixels, maintain higher framerates, and in 2D use at 4K you would want a video card with displayport or HDMI2 for a 4K display to have the bandwidth to use a higher refresh rate.
Then again if your game supports that aspect ratio you can just set the video driver and monitor to not scale it and use the 1440p resolution with black bars on a 4K monitor.
The black bars might create the mental anguish that you're wasting some of the monitor's abilities, but you are only losing the same you would if you'd picked the 1440p in the first place, while still having 2160p for other uses.
You could find a store that has display models at both res. and compare them.