You are talking about 2 different sized screens so we can't compare resolution directly. I'm sure 4K on a 43" monitor will look sharp and 1440p on a 32"UW will look sharp. I'll leave it to someone else to do the ppi math. Bottom line is they will both look really good resolution wise.
It also comes down to personal preference. To me, having a 43" "monitor" would look silly, like you have a tv on your desk. I'm also not big on looking up and down so much on a monitor... the 43" will be much higher.
The ultrawide looks sleek (personal preference) and would also be a lot more useful for computer applications like snapping two documents to either side. Also widescreen movies will be better.
Really all personal preference. But if ou gave this choice to most people who know what they are talking about, I think 90% would prefer for the ultrawide