The i7 5820k is overkill for gaming, even the i7 6700k is unnecessary for gaming. If you really want a overclockable i7, I would recommend the i7 4790k. This will be good at least 3-5 years into the future. However to specificly answer your question, the i7 6700k would be better because it is clocked at 4.0 GHz, while the i7 5820k is only clocked at 3.3 GHz.