geofelt :
buy a z370 motherboard, a 8700K and don't look back.
If you do anything less, you will forever be second guessing yourself.
I have rarely regretted paying top price for something good.
I have often regretted settling for something cheaper.
I would say the same, but also add that I bought an i7-7700K for top dollar wanting the best; and less than 6 months later the i7-8700K was released. But that will always be an issue with new hardware being released.
Btw, i7-4790K is a different socket than OP's i5-6600.