That's a tough call. as the 'mainstream' CoffeeLake will either be out 'soon', or, in Feb 2018, depending on who you listen to or believe. We are not even truly certain which socket it will be on, save for one leaked CPUZ screenshot that seemed to imply there was at least one sample on a socket 1151...
However, some unverified 8700K specs leaked, showing 3.7 GHz base/4 GHz turbo clock? We will have to wait and see if that will even defeat the current 7700K in most games out now....
One things is for sure...
I *personally* would not choose any 4c/4t (6600/7600 or below) solution today....
Either go with 7700K, or investigate the R5/1600 or above.
(Too many folks with i5 processors reporting slowdowns, stutter, inconsistent framerates, etc., with newer gaming titles...)