Which gives better mileage- 87 or 91 octane gas?

I saw that ethanol was used to boost the octane rating of regular gas to 91. Since ethanol gives less energy than gasoline I was thinking the higher octane rating would give me worse mileage. But then I thought ethanol contains more oxygen than gasoline, so it might burn more completely and give more mpg. Its a long and boring ride cross country without a radio to keep my mind occupied. Any thoughts?