Of course depends on budget, but if improving gaming fidelity is something that's very important to you then a gaming monitor is the way to go. A Gsync for Nvidia or FreeSync monitor for AMD, though expensive, greatly enhances responsiveness and reproduces visuals most faithfully.
A 27" montor at 1440p sounds like a step down in size, but trust me it's so much easier on the eyes to look at that sitting closer isn't an issue. Sitting closer, and having the wider field-of-view of the 2k resolution, makes for a great experience.
And because 27" 1080p Gsync monitors and 1440p are so close in price, I would recommend going with the 2k resolution if investing that much in a monitor. The only issue I see is that a single 970 will not be able to fully drive that at 120+ fps without some compromises. You could always display at 1080p on the monitor for a work-around, but eventually you'll want to at least add a second 970 to get the most out of the experience.
[/my_two_cents]