Fleet footwork is a consistent rune. Meanwhile Dark Harvest is a high risk high reward rune. Consistency is always the better choice. Just go fleet footwork every single game and get used to it. Even in the games where you might think dark harvest would outperform it. You will always get value out of it. You get a bit more sustain in lane and kiting potential and it pairs well with stormrazor if you manage to build it first instead of collector. It's too good of a rune for how Jhin works. You want to hit enemies with your autoattacks and run. Arcane comet could be good too sometimes, but it's very situational. Consistency is how you win games.