it depends on what you want to do with it
depends on which tasks you need it
difference in games isn't that big but if you are not on budget go for i7
i7 offers 6 threads extra in comparison with i5 and maybe better oc potentional
do you do streaming, rendering, work with photoshop, adobe? if no, then 8600k is enough.
there is only a couple of fps difference in games
i7 is not needed for gaming, unless you really want it and can afford it. I'd just buy the i5 and save money.
Intel is about to release an 8-core CPU by the end of this year, I would wait.
8600K doesn't have the hyper-threading, it will become shit faster than 8700K, it's important for CPU-heavy games like Battlefield, GTA, Fallout, and it's important for streaming.
Personally I would go for Ryzen 2600X.
What about a motherboard? If you are building new PC, maybe think about investing into one you can overclock your CPU on.
They have pretty much same performance on games go for i5 if your not gonna stream
either i7 or Ryzen (AMD)
i3 and i5 not worth anymore these days without hyperthreading