Well, the 6700K has four additional threads—additional sets of instructions that your processor switches to whenever it's idly waiting for information it needs to execute its standard set of instructions. No amount of overclocking is going to add those extra threads.
However, whether or not those additional threads make a difference is wholly dependent on the game/software you're playing/running. Dolphin, for example—which is essentially always CPU-limited, basically can't make use of these extra threads at all. Furthermore, the 6600K can often be overclocked further than the 6700K in terms of actual clock speed (how quickly it can execute instructions).
In other words, it depends. Overall, the 6700K is generally a better performing processor than the 6600K—if that wasn't the case, no one would ever buy it—but that isn't always the case, and it costs an extra $100.
TL;DR Get the 6600K.