If you're talking about pure efficiency as energy (in BTU's) removed from the conditioned space vs. energy spent on running the ac, then having the fan off is slightly more efficient.
But if you're comparing dollars spent on electricity for the same comfort, leave the fan running.
With the fan off, air tends to stratify, with the cold air down by your feet and the air up at your torso and head being warmer.
But if the fan runs and the air keeps circulating, this doesn't happen. So you can be comfortable at 76º instead of 72º and that saves energy.