From my experience: 16:9 keep it flat. If your going fro 21:9 and your sitting directly in the center of it (not viewing it at an off angle.) get a slightly curved panel.
I have a LG 34UC89G-B (3800r curve), a LG 34UM57-P (flat), and I had a BenQ XR3501(2000r curve).
The sides of the 34UC89G don’t look any closer or farther away, then the center of the screen, but the sides of the 34UM57 defiantly feel like they are farther away. The only way that I notice the curve is if I’m looking at the 34UC89G from a birds eye view, other wise it appears flat, but it has one of the tamer curves, that I’ve seen. On the other hand, the XR3501’s curve was more prominent, especially if I was slightly off angle.