I was kind of in the same situation. I think it really depends on lots of things. In my case the main consideration for me where the future, I think we will see quite a jump in CPU performance with AMD coming back and some other factors making for higher demand in terms of CPU power. ¨
So I plan to sell the thing after two years max. when we see mature Ryzen or Intel chips with new architecture. Now for a range of 2 years an i5 is more than enough, I'm not going to need the extra threads for gaming until then.
In terms of whether to go for 8400 or 8600k I think it's really a question of Overclocking, as far as I've seen the 8600k OCs quite nicely, matching the performance of a 8700k in gaming, which for me was the nobrainer argument. So if you don't plan to OC don't go with the 8600k, if you do I think it's a great deal.