I own a 144hz 1440p gsync monitor, 980 ti hybrid, and an I7-5930K (no overclocking)
I run overwatch with gsync off, and graphics at the lowest settings to get my in game fps maxed out.
Overwatch mouse lag is unbearable, and you need a high framerate for the game to be worth playing. So if by "flawlessly" you mean be able to aim smoothly, without a floaty laggy experience, then go with 1440p. Similarly if you are purchasing the monitor for more than just overwatch, absolutely go with 1440p. If you want the highest graphics settings, but also want smooth gameplay, 1080p might be a reasonable option.