Hey OP. I can answer this question more specifically because I own a pair of 980s with custom water cooling. I recommend 1440p. ESPECIALLY if you spring for an Acer predator XB1 which is what I have. You get up 144hz (or 165hz with easy OC enabled), IPS display, and a gorgeous matte finished panel.
They key here is that 1440p is a sweetspot, your 980s will get great framerates without the need of anti-aliasing. I play BF4 (twitch shooter/tons of explosions,etc) and my settings are cranked except for anti-aliasing. At 1440p there's already enough pixels to the extent you don't need antialiasing because it's naturally removed.
4K is great if you want screen real-estate but bear in mind your games will suffer greatly unless you lower settings, now if you had a pair 1080s I'd say it'd rock both 4k or 1440p like a beast.
Sell your 4k panel, get this bad boy. I love it and know you will too- 1440p is also alot easier for scaling things like text, web browsers, etc.;
EDIT: I forgot to mention, most importantly this panel is also G-sync enabled so you won't get stutters or screen tearing for the moments you drop FPS quickly.
http://www.amazon.com/Acer-Predator-XB271HU-27-inch-Widescreen/dp/B0173PEX20/ref=sr_1_1?ie=UTF8&qid=1463942660&sr=8-1&keywords=predator+xb1