Here you can find a comparison of both CPUs:
http://cpu.userbenchmark.com/Compare...
https://ark.intel.com/compare...

On the data sheet, the two differ mainly in the following points:

- Number of Cores (i7-7700K vs. i5-6600K)
4 vs. 4

- Number of threads (i7-7700K vs. i5-6600K)
8 vs. 4

- Processor Base Frequency (i7-7700K vs. i5-6600K)
4.20 GHz vs. 3.50 GHz

- Max Turbo Frequency (i7-7700K vs. i5-6600K)
4.50 GHz vs. 3.90 GHz

- Cache ["SmartCache"] (i7-7700K vs. i5-6600K)
8 MB vs. 6 MB

- Intel Optane support (i7-7700K vs. i5-6600K)
Yes vs. No

- Intel® Hyper-Threading Technology (i7-7700K vs. i5-6600K)
Yes vs. No

I hope I could help you! : )