First of all: Coffee Lake requires DDR4 memory, not DDR3.
When calculating, keep in mind that the I7-8700 includes the Intel basic heatsink and the I5-8600K does not include any, so if you do not have one, you should buy one.
Speed:
I7-8700 base 3.2ghz turbo (4,5,6 cores 4.3ghz) (3 cores 4.4 ghz) (2 cores 4.5 ghz) (1 core 4.6 ghz)
I5-8600K base 3.6 Ghz turbo (5,6, cores 4.1 ghz) (2,3,4 cores 4.2 ghz) (1 core 4.3)
The clock-base has no practical importance because the processor uses the turbo without the need for any action on your part. The important thing is the turbo of 6 cores and 1 core. In both cases I7-8700 wins. If you are not going to overclock the I7-8700 it is much better.
That said, both processors are much more powerful than your graphic card requires and, unless you also perform cpu-intensive tasks (rendering videos for example) you will not take advantage of them.
Any processor from I3-8100 would be sufficient, but since you want your system to last as long as possible I would recommend the I5-8400.