If you're dead set on investing in a Gengar, which is totally cool because Gengar is a sweet pokemon, I would prefer the Sucker Punch one. The Shadow Claw variants (legacy) are the best in terms of DPS, but Sucker Punch / Shadow Ball is about 91% of the damage of SC/Shadow Ball.
Pretty good mon you got there.
EDIT: you can check out the comparative DPS of Gengar movesets at this page :::::
http://pogomoves.com/check.php?lang=en&pkmn=Gengar&basic=Shadow%20Claw&charge=Shadow%20Ball