League of Legends (LoL) Question: Electrocute or Dark Harvest for Evelynn?

Electrocute without question. The only selling point of Dark Harvest is the fact that it refreshes on each kill, and Eve has no way to use that since she can only take out one target per fight. Basically there's no downside to the 20s cooldown of electrocute on Eve which is its only shortcoming.
You might be thinking DH is more damage late game but even this is not that obvious, since electrocute is 220 base damage at lvl 18 (same as DH with 140 souls) and has a higher AP scaling.

If you are confident on taking a lead, go for Dark Harvest, but it depends on what the other jungler chooses, if you get counter jungled early game its going to be very hard to make it up

The issue that you run into is that Dark Harvest is much better for the late game. After 150 souls it does a ton more damage than Electrocute. What most people don't take into account is that you do not want a late game Eve. Evelynn's strength is that she snowballs early and mid game and is able to delete champions. She is not a great champ to have in a teamfight, and she has no real escapes. As you get later in the game, she loses her advantage and will become less and less relevant. As such, I would recommend Electrocute for the early damage and the shear burst.

Really depends on your playstyle. Dark Harvest is for power farmers and those who want to scale almost infinitely in the game. Electrocute gives you a better early to mid game but falls short later on. That's all.

I take Dark Harvest cos in low elo Evelynn is broken anyways and you can get fed so easily but the higher you go the more useful Electrocute becomes