One thing I've noticed since the price of regular fuel has dropped, is that there is a much larger gap between 87-91-93 octane fuels. It used to be generally 20 cents from 87 to 91, and another dime for 93. Now its 35-45 cents from 87 to 91 at most stations around here. There is one station though that was 10 cents between 87-91-93, so 93 was only 20 cents more than 87, and they have maintained that gap so far. Guess where I go when I want a tank of 93.
On my Eco, I don't see much difference in 87 and 91, in either power or mileage. There is some, but not enough to make me use 91 all the time. I see a significant difference in 93 over 87 though.
I'm just glad I'm not buying diesel fuel, at twice the price of 87. (1.79 vs 3.59). Ouch.