With Skylake now available, there is absolutely no question in my mind that a new build should be Skylake.
a. Prices for cpu, z170 motherboard and ddr4 ram are almost precisely the same.
b. 6600K has an estimated 5-10% performance improvement per clock over haswell.
c. 14nm runs cooler, you get a decent overclock without the need for exotic cooling.
d. 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
e. skylake can be upgraded in the future to kaby lake.
The i7-6700K is a fine processor, but due to demand, there is price inflation.
Actually, for gaming the 6600K with an overclock will be equally good.
Most games can only use 2-3 cores, so the extra I7 hyperthreads will be unused.
Here is what you can expect for skylake overclocking.
What percent can get an overclock at a somewhat sane 1.40v
I5-6600K
4.9 3%
4.8 23%
4.7 44%
4.6 85%
I7-6700K
4.8 12%
4.7 47%
4.6 91%