You might catch some jazz for asking this question. It has been asked and answered on here dozens of times. Same with "How do you disable the reverse beep?" and "What are the best tires for the Prius?" The search function actually works fairly well on this site. Happy reading.
Pound for pound, there is pretty much no difference. However, some companies like to put 10% ethanol on 87, and 0% on 91 - hence why you might see an mpg boost with 91 over 87. Shell is like that...at least up here in Canada. You might want to check with your local gas station to find out.
I'm sure someone on here has figured this out, but is it cheaper to pay $.10 extra for 87 octane (like we have to here) over 89 octane to avoid the ethanol since the ethanol blend tends to reduce fuel economy?
