You mean they'll improve the GP104 drivers. Whereas the GM200 drivers are basically fully mature already.Definitely go for a 1070 over a 980Ti. You KNOW it's a matter of time before Nvidia gimp the Maxwell drivers and your 980Ti will be way worse than the 1070. Sad but True.
They don't 'gimp' anything. :/ Nvidia tend to be better with having more mature drivers on Day 1 so there's not always constant room for improvements years after release.