Will you be adding a second 970 eventually? If so, then either would be great....but 21:9 is great way to get a better FOV while gaming.
Either way you do go though.....keep in mind that 1440p has twice the pixels of 1080p. The 970 is a wonderful card for 1080p gaming, but at 1440p it will require setting and detail compromises to keep your FPS up. If the most important thing is eye candy, then you may find with a single 970, you get more details than are possible at 1440p (of course, game dependent).