The only difference between 8600k and 8700k is the hyperthreading.
The cost difference is quite big.
For pure gaming rig, 8600k is sufficient.
Vast majority of games will run a tiny winy bit better on 8700k and only few can run noticeably better. The difference is still quite small, if you consider the extra price.
If you do more stuffs beside gaming, 8700k can be the better choice.
The future proof for HT, well..it is still a gamble, if you do not need really it for your normal usages.