Just read this interesting comment on Reddit regarding a discussion on laptops. Any thoughts? Of course there are no 16:10 HiDPI desktop monitors, so the choice is really between 16:9 and 3:2.
16:10 / widescreen is brilliant productivity-wise for work that involves side-by-side mutitasking. Want to open two docs, two webpages, two spreadsheets, etc. simultaneously? Widescreen is infinitely better. But that only works if the screen is big enough for multi-tasking at high resolution (e.g. 1080p @ 100% scaling minimum) to be comfortable - for me that's 15.6" or bigger.
On a 14" or smaller screen, it's too small to comfortably side-by-side multitask, so because you're inherently "limited" to a single open window at a time, 3:2 is much better. Alt+Tab is your best friend then.
This is also why I would never want a 3:2 external monitor - any desktop external monitor is going to be big enough to invite comfortable side-by-side multitasking, so widescreen is obviously superior.