Honestly, It comes down to personal preference.
I'm gonna just come out and say that i'm biased towards sucker punch, as I was traumatized by gengar several times. Also depends on your team, here's what I reccomend:
If you're going with it as a late-game sweeper, use sucker punch.
If you're using it as a wallbreaker or to put pressure on certain Pokemon, use crunch just for Pokemon like gorgeist.