something I learned yesterday from a arson investigator...gas stations 90% of the time either have too much gas being delivered or not enough to make it through the day. It is very common for them to share gas, and since their tanks are so big, most of the time it is not filled with one "brand" of gas. This is also why when they investigate arson where gas is involved, they cannot determine which station it came from.
With that being said, I would stay away from the small stations because they are looking for the cheapest thing they could find at the time, but for the rest of the big companies, there really is no difference from one to the next.