If you're thinking of upgrading from a 980 Ti to 1070, don't. At best they're equal, but I think the 980 Ti is generally faster. At least it is if you have an aftermarket 980 Ti.
The reference 980 Ti is awful. It's like reference Hawaii bad. While reference Hawaii was hot and loud, the 980 Ti isn't as bad in those areas, but greatly damages performance, more than the cooler on Hawaii did.
According to TPU, the 980 Ti at stock gained 9.4% performance from overclocking to 1437/2070, an aftermarket 980 Ti overclocked to 1432/2020 is 23.2% faster than reference, even though its maximum clock speed is slightly lower. The best aftermarket 980 Ti is 34.4% faster than reference.