The speeds you listed are the base clocks, there's also the turbo boost clocks. The 6500 goes up to 3.6ghz, the 6600 goes up to 3.9ghz. The reason the 6600k is a bit more expensive, it's unlocked and has the ability to be overclocked. It's not limited to 3.9ghz, overclocked it might reach 4.6, 4.7ghz. Of course it also involves a better cooler and motherboard but that's the reason for why it costs 'a lot', it has the potential to be a lot faster.
Since you said you don't want to oc, there's no reason for getting a k series cpu and paying the slight premium (currently $4 usd more than the 6600 non k). If you need to save $15 or don't think $15 is worth 300mhz per core extra then the i5 6500 would be the better choice at around $200 usd...