I'm in the same boat currently, but strongly leaning toward the 9700K. That said, I think it's useless to try and speculate as to what will happen down the road. Right now, and likely over the next few years, there are trade-offs between the two depending on the particular game or app, rather than one being full-stop 'better' in performance. Some like more cores, some like more threads. This isn't going to change any time soon.
The main distinction here is that the 9700K has a bit better temps (esp. if OC'ing) and power efficiency, and has hardware-level patches for some (not all) of the Spectre/Meltdown stuff instead of OS-level. If that matters to you, go for it. The 8700K is undoubtedly a better value overall, but runs a bit hotter and Spectre/Meltdown patches are implemented only at the OS level.