Then go with the 1070 if you play fast graphics games like Battlefront or cs:go or gta:v, the difference between 60Hz and 144Hz is considerable and while the 1060 can do it sufficiently, you'll end up in the same boat you are in now much sooner than wanted.
As far as bogging down that i5, that's not on the gpu, but upto the game itself. Invariably you are going to start running into games that take advantage of multiple threads, like the 8 threads offered by an i7 or Ryzen cpu. This is where games are heading more and more, so what might hurt is you have an i5, not that it's slightly older. I already see a difference in my i5-3570k and i7-3770K and they are intrinsically the same cpu, one with HT. But it'll be several years yet before...