Few games can use more than 2-3 threads, so the i7 hyperthreads will hardly be used, let alone a 6 core cpu.
More cores will reduce the oc capability per core.
It is the single core performance of the master thread that matters most in gaming.
A I506600K or a I7 6700K with an overclock is as good as it gets for gaming today.
They are really not far apart in performance.
If budget is an issue, save the $100 premium for a I7 for use elsewhere.
As of 5/2016
What percent can get an overclock at a somewhat sane 1.40v Vcore.
I5-6600K
5.0 2%
4.9 11%
4.8 36%
4.7 64%
4.6 88%
I7-6700K
4.9 2%
4.8 17%
4.7 59%
4.6 93%
4.5 100%