Z170 is for a Skylake cpu build.
Z97 is for a haswell build.
With Skylake now available, there is absolutely no question in my mind that a new build should be Skylake.
1. Prices for cpu, z170 motherboard and ddr4 ram are almost precisely the same.
2. 6600K has an estimated 5-10% performance improvement per clock over haswell.
3. 14nm runs cooler, you get a decent overclock without the need for exotic cooling.
4. The Z170 chipset permits the use of much faster ssd devices on the horizon. Samsung 950 pro for example:
http://www.samsung.com/global/business/semiconductor/minisite/SSD/global/html/ssd950pro/overview.html