So far the 9700k only offers a slight gaming performance boost over the 8700k, and that seems to come down entirely to the higher stock clock speeds. If you overclock the 8700k to the same speed as the 9700k, the chips are functionally identical in gaming under most circumstances, mainly due to virtually no games scaling across more than 6 cores effectively. As for overclocking the 9700k, the new 9th gen chips don't seem to have much in the way of headroom in terms of clocks or even thermals. The solder doesn't seem to have really helped thermals much, likely due to the core count increase counteracting that. You may get slightly better overclocking results with the 9700k, or you might not depending on the silicon lottery and whether you have a very high end cooling solution.
At this point, it probably comes down to which chip you can get cheaper. Both the 8700k and 9700k will offer very similar gaming performance, with the 9700k holding a slight edge if you have absolutely no intention of overclocking.