Gas grade - 87, 89, higher?

[Bostonian]

Update since my original post: Tracking the mileage performance on 87 vs 91, I notice maybe a half mile per gallon improvement, but no other noticeable differences. What you all say makes sense about higher octane/performance, but I'm still somewhat sceptical. Also, the mileage per gallon improvement also coincided with a fuel injection cleaner I used for that tank.