I had the very same doubts. I think 21:9 is better for games, since easier for GPU (less pixels) and glorious for fps and driving games, while 1440p is better for contents (I came from 1536p, 1080 is way too low) and general compatibility. That was a draw, so I decided to consider a third element: refresh rate. I bought a Korean ips in order to reach higher rates.
Since IPS defects are mainly lag and ghosting, I thought higher refresh rates would help