http://gpu.userbenchmark.com/Compare/Nvidia-GTX-1080-Ti-vs-Nvidia-GTX-1070/3918vs3609
There's a difference, definitely. Whether this difference is worth the money to you, is for you to decide. 55% is a noticeable performance increase, for sure. What is your full PC config, including monitor/s?
If you have a 1080p 144 Hz panel, a GTX 1080 will be enough, but a 1080 Ti will, of course, be more future-proof. If you have 1440p 60 Hz panel, again a GTX 1080 is fine, but a 1080 Ti will of course play at higher details, and will last longer. If you have a 1440p 144 Hz panel, then a 1080 Ti will be absolutely worth the upgrade, similarly for a 4K 60 Hz panel.
On the off chance that you have a 1080p 60 Hz panel, don't get the 1080 Ti, unless...