The cpu will be used for functionality, gaming and productivity.
I think the cost will be higher for building the core i5 6600K due to more expensive z170 not including the cpu fan/cooler. Wattage and heat is also higher possibly stressing out the i5 6600k cpu faster. Speed will be nearly the same. But with a lower cost I get more features on the i7 due to hyperthreading, and the cpu runs on base clock with no worries of stressing out the system too much.
The cost is higher for building i5 6600K on z170 than i7 6600 on h170, is it not?