The difference is that a higher octane fuel generally encourages an increase in 1-2 MPG, Increased Throttle response & smoother power transitions, reduced risk of knocking or pinging, and reduced risk of Horsepower loss from heat soak.
The cost for this above is approximately .20 more, if you are comparing the lowest octane to the highest octane, generally available at the gas stations.
For more information on an 87 Octane Vs 91 Octane, a Report is available here from a random study.
