Which format would you choose? Both 21:9 and UHD offer their unique characteristics but is one better than the other? Yes… but that depends on usage.
For 4k screens, one great pro is that with these type of screens, users can not see any pixels unless they press their face against it. One complication is that most of these screens can only go up to a max refresh rate of 60Hz which many gamers may dislike. Another problem is that the icons and taskbar will be minute forcing some to squint and strain their eyes.
When comparing the Ultrawide to the 4K, it is generally better in terms of productivity. Movie editors will love this as they get a huge horizontal screen real estate expansion. Different than the 4K, these type of screens do not have the minuscule icon problem which is wonderful. When users watch media content, they will be blown away as it is so immersive making them feel it is life-like. Ultrawides can excel beyond 60Hz as it is at a lower resolution. For youtubers, this is a no-no because they require their lines to be straight so they can see their composition correctly.
An annoying issue is that when viewing videos, there will be black bars on the sides of the Ultrawide. On the 4K, the black bars will be on top and at the bottom.
Overall, most should get UltraWides as gamers disapprove of low refresh rate and not many single GPUs can handle it.