Which is a good move?
Shadow ball should likely come Standard for gengar, seeing as he gains STAB with it. Dark Pulse covers the exact same types as Shadow Ball, so it's to your advantage to go with shadow ball.
I think both because even though shadow ball is STAB, dark pulse can flinch, so I actually use both, but if you want a troll gengar, dark pulse is better, if you want a special sweeper, shadow ball is better. its ur choice really.