You want only one Monitor? not 3 ? Are you sure ?
If you want it for gaming go with the 21:9 ! The extra FoV is worth it (IMHO!)
They are both IPS monitors, so the image quality and color should be quite good.
On the other hand does the Asus 49 - 86 Hz refresh rate. So maybe you can overclock it from 60hz to 75hz or 85 hz. This would result in a smoother gameplay.
As for your fear of stretched images.
This only happens in games! Most Programms and web pages do not stretch the buttons/texts but only move their position.
For only streams with Flashplayer / HTML5 or Videos with VLC they do not stretch! They're all Pillar/Letterboxed and will have black borders.
Maybe someone who has a 21:9 Monitor can give you more detailed information on what works and what does not
Oh and yes. 2x 290X is totally Overkill for 2560x1440/1080
btw: