For multi-tasking the best setup is 2 monitors, as you'll have an easier time organizing your tabs between 2 monitors than you would one big monitor.
For example; if you want to have 4 different programs open at once with 2 monitors you could just drag them to both sides of one monitor, making both programs take up half of monitor 1 consecutively the same thing with monitor 2.
Whereas with a ultrawide screen youd have to resize every tab individually every time you open a new tab which could be tedious.
However, its well known that a ultrawide screen generally makes video games look amazing when they take up the entire monitor.
In short: If you constantly have multiple apps running that you are working on at the same time, buy a second monitor.
If you like to game and want your gaming experience to look amazing, buy an ultrawide screen.
Good luck, hope this helps.