Performance will be very similar between the 2, one of main advantages of the 1070 would be lower power use, if you pay the bills and use it a lot you may save the difference in initial cost over the life of the card 2-3 years (length of guarantee, it will last longer) if you use a it a decent amount every day with a lower electric usage.
Saving of 100 W per hour under load, 5 hour use a day will save £20 a year, 3 years = £60.
If you use it less it's not going to as much of a feature. If you use it more then it will become a major factor.
I'm not sure what the noise factor would be in the 980ti, but I suspect the fans operate all the time, where as most of the partner cards of the GTX 1070 will stop at temps below 60 - 50c depending on card. Which is very nice if you have the PC on a lot when your not gaming.