Check every benchmark b/n the i7 8700k and the i5 8600k and the i7 barely outperforms the i5 and in some benchmarks the 8600k does better. i5 is the sweet spot not worth paying an extra $100 bucks more just for the i7 bragging rights.
8700k will make your pc a bit more viable for 1-2 years longer then the 8600k due to its extra threads. So longevity and performance go i7, but just for gaming and are willing to upgrade in like 2 years go i5.
Most games don't use hyperthreading and are GPU bound more than they are CPU bound. If you're looking to get good performance and save some cash go with the I5. If you want a high end, balls to the walls system go with the I7.