heres a good idea of how much more powerful the gt x1080 ishttp://gpu.userbenchmark.com/Compare/Nvidia-GTX-980-Ti-vs-Nvidia-GTX-1080/3439vs3603
you get roughly 25% more performance from the gtx 1080 however if you play CSGO, world of tanks, world of war craft or esspecially league of legends you should stick with the 980.
if you can afford the gtx 1080ti it 60% more power compaired to the gtx 980ti