Traditionally the mid grade stuff is the worst deal overall. I agree that if you notice a difference then look to the gas quality or engine issues. Maybe you are getting some winter blend that is effecting your performance?
I agree with the above. ^^^. Some winter blends don't seem to be as good when they went to the 10% Ethanol blend. Maybe just me, but our Escort was sensitive to the blends of differing branded gas. It was always stuttering on Exxon and Texaco. Burned the QT brand and RaceTrac just fine. Those are discount brands as well. It also did better mpg wise at the discounters... :shrug: go figure.