The decision has many points: power consumption, heat generation, DX11 performance, DX12 performance, and of course price.
Generally speaking, AMD generates more heat, consumes more power, and runs less expensive for similar performance. But this very much depends on exactly which GPUs you are comparing.
If you're after top performance, the R9 Fury cards are beating the 980Ti in many benchmarks. Again, consider the power and heat issues AMD GPUs create.
Lastly, Nvidia will be announcing their next top-of-the-line GPUs which are currently scheduled to be released in June.
Set your budget. Figure out what your PC can handle power-wise, how it can handle heat, then consult many benchmarks for settings and games YOU would use. Last is price shop.