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New monitor definitely.
1440p will look significantly better than your current monitors and the 980 will still perform good enough to keep you satisfied.
Upgrading your 980 won't actually give you any real visual treat. You will get a higher framerate but since you are maxing your games anyway I doubt you could honestly be that excited by it.
The jump to 1440p or 4k is far more satisfying. You will feel a significant change for your money.
1440p will look significantly better than your current monitors and the 980 will still perform good enough to keep you satisfied.
Upgrading your 980 won't actually give you any real visual treat. You will get a higher framerate but since you are maxing your games anyway I doubt you could honestly be that excited by it.
The jump to 1440p or 4k is far more satisfying. You will feel a significant change for your money.