Shield/Dark or Shield/Electric

/Dark Melee is mostly a single-target set that would improve survivability. It provides a self-heal in Siphon Life (very good for Shield/ which has no heal of its own), and its secondary effect is a to-hit debuff, which will make your Shield/ primary that much more effective. You also get an End Drain/Restore power in Dark Consumption and a slower-recharge damage buff power in Soul Drain that also does some AoE damage. Negative Energy is also less resisted, but it's not as much of an advantage over Electrical as it is over Smash/Lethal.

/Electrical Melee is much more AoE-oriented, with four AoE attacks. Lightning Rod is a great power. The end-drain aspect of Electrical Melee can be good, but without something like Power Sink and Lightning Field as a base for sapping I don't think you can count on it as an effective mitigating effect.

Really, it depends on what you want to do. If survivability and single-target damage is your primary goal, /Dark is better. If massive AoE damage is your goal, /Electrical is better.