Which gengar should I power up?

It definitely makes a huge difference, I have a maxxed 89 Gengar with SC that outperforms my maxxed 96 Hex Gengar. But since your SC is so bad I don't know which would come out on top there. Then again, assuming the 7 is defense it probably would still be better.

You could just punch some simulations in to see the difference yourself:

GoBattleSim vs C/FB Mewtwo, Gengars L40, no friendship or weather boost, no dodge, 1000 simulations averaged:

SC/SB Gengar: DPS 17.5; TDO 1.9% (~237HP)
H/SB Gengar: DPS 14.74; TDO 1.57% (~196HP)

Even with the significantly higher IVs on the Hex Gengar the Shadow Claw Gengar has much higher DPS and higher TDO on average than the Hex one. If dodging is your thing Shadow Claw is much easier to dodge with as well. There's a reason SC Gengar is a fan favorite.

EDIT: Those sims were done with the IVs you posted above, not an assumed 15/15/15 on each.