I guess it depends on what you're spiritomb is invested in EV wise! Is it a sp. atking? Or Physical atcking?
Of course just go for whatever fits your play style and hopefully the one that fits your spirittomb! Of course if you have a physcial spiritomb sucker punch, otherwise dark pulse!
Personally I'd run both of them, but it depends on your other moves!
If you play tactically give your spiritomb the kings rock and Dark-Pulse it is!
If its a slow Spiritomb, and you play very aggresively, go for sucker-punch!