Both CPUs offer the same stock boost clocks, so if you're not overclocking, then the 9600K doesn't offer any real advantage. The 8700 has Hyperthreading (SMT) though, allowing it to more efficiently run a second thread on each core, which should give it the performance advantage at heavily multithreaded tasks. Most current games will perform very similar on either, but games are likely to become more heavily threaded down the line.
Also, the 8700 comes with a stock cooler, while the 9600K does not, though if you want the 8700 to push it's maximum multi-core boost clocks, you may want a cooler for that as well. Even a $30 tower cooler would likely be enough to keep it operating at full performance.