Competitive wise, they're both almost the same. But, with these Natures, you should go Sableye. This nature is not good for your Gengar to use it to it's full potential. (You can still use it, you just won't be able to have as many KOs as another Gengar would.)
Sableye@Sablenite
EVs: 252 Def, 128 Sp. A, 128 Sp. D
Nature: Bold
Ability: Prankster-->Magic Bounce
-Will o Wisp
-Calm Mind
-Shadow Ball/Dark Pulse
-Recover
Will o Wisp is to make sure you're not damaged too much because of the Attack lowering, Calm Mind is to boost your Special stats, Shadow Ball/Dark Pulse for STAB, and Recover is to make sure you're alive.
Even though your Sableye is better, you may still be interested in breeding for another, better Gengar.
Gengar@Gengarite/Black Sludge
EVs: 252 Sp. A, 252 Spd, 4 HP
Nature: Timid
Ability: Levitate (if Mega-->Shadow Tag)
-Shadow Ball
-Sludge Bomb
-Giga Drain
-Dazzling Gleam/Focus Blast
Shadow Ball and Sludge Bomb is STAB, Giga Drain is coverage just in case, and Dazzling Gleam/Focus Blast (Focus Miss!) is coverage for Dark, Reliability vs. Power. No matter what item, the set is the same.
Hope I helped!
