Either scene alone would be sufficient for proving the Oscillator operational, but each has additional benefits. The Snoke scene allows for the Ileenium system as the new target, so it would allow for Hux’s ‘prepare to fire’ line to be included. The Snoke scene also solves the ‘new star’ problem by implying that the planet has enough juice for another shot, something that the Ackbar line doesn’t imply.
On the other hand, the Ackbar scene nixes the argument that the X-wing attack is now just an act of vengeance, especially if it is coupled with the Snoke scene.