Okay so, here we go:
Type of Move: Both Physical, which Golisopod is excellent in.
BP: Leech life with 80, plus it's STAB(Same Type Added Bonus), which adds 50%
Accuracy: Tied
PP: Leech Life with 10
Sucker Punch does have Priority though, which is helpful with his low speed.
Special Effects: Leech Life wins, as Sucker Punch's Effect is actually bad, and Leech Life heals, which can be helpful to counteract it's bad ability Emergency Exit, plus if it's holding a big root, the health regen can be boosted, the only downside is with Liquid Ooze Pokemon, Tentacool and Tentacruel(Gulpin Line's not in Gen 7).
Typing effectiveness in general: Sucker Punch
So, when you add all this together, combined with the fact that there's still 3 other slots and plenty of Dark moves that it can learn through TM or Tutor, I'd go with Leech Life.
Source: A lot of cross referencing between the Type Chart, and the two Moves.