If you use the Acer monitor, it's only 60 Hz. So if you're only aiming for 60 fps, I'd say a 4690k should be fine for the most part. If you use the asus and want to take advantage of the higher refresh rate with higher fps, CPU power becomes more important so you might want to get an i7. That being said, a 980 Ti probably won't be able to get more than 60-80 fps at 1440p/max settings in AAA games anyway.
So the optimal CPU for your 980 Ti comes down to which monitor you want to use, and which games/settings you want to play.
Edit: Do you already have DDR3 as well, or just the Z97 mobo?