It would be interesting to see how an 8 core/8 thread chip would fare against a 6 core/12 thread chip. If you assume that hyperthreading is about equivalent to a third of a core performance wise (this is a best case scenario) then the 8700k might theoretically be about as fast as the 8 core/8 thread chip (rumoured to be the i7 9700k). The 8 core chip would do better in things that use lots of cores but don't benefit much from hyperthreading, and the 8700k would likely have the edge on less threaded tasks due to likely being able to be clocked higher and tasks that use hyperthreading quite well may have the 8700k perform very close to the 8 core/8 thread CPU.
I suppose you have to ask yourself how urgently do you need the upgrade. If you need a computer right now, then just go with the 8700k or maybe look at the R7 2700 if your software would benefit from an 8 core CPU. If you can afford to wait a couple of months for the Coffee Lake refresh, you can wait and see how things shake out in benchmarks. All I can say is I wouldn't be surprised if the 8700k retains the gaming crown due to higher clocks, and in heavily threaded productivity tasks, the 8700k may offer similar to slightly slower performance than the rumoured 8 core/8 thread model.