Sucker Punch
While Night Slash is a great option (depending on the rest of the moves), Sucker Punch has that all important +1 Priority which pushes things in its favour. Since both are STAB and have a very small base point difference, the damage output is negligible, unless you count Crit hits (which is not a bad idea since Night Slash has high Crit hit rate, but I generally don't want to rely on chance.)
That said, chance is involved as your opponent might not use an attacking move. This is kind of rare since Bisharp is quite frail with a 4x weakness to Fighting and a weakness to Ground (Close Combat / EQ being extremely common). Even in-game, most attacks are damaging. While Bisharp is decent in terms of speed, Sucker Punch is very important against the faster threats in the meta.
Finally, it is important that both are good moves, with Sucker Punch important against offensive opponents and Night Slash against defensive ones. Based on the ratio of teams you face (Generally offensive is higher, hence the preference), you can go for whichever.
Tl;dr Priority > Power unless you prefer Crit hit hax.
