In my area it's about a 5 cent difference. I remember 5-15 cents in the San Jose area. It could just be based on cost. I've noticed the appearance of Arco stations and workers is usually a notch below that of their major competitors. As for corporate efficiency, I have no idea.
The gas is the same. Between Chevron, 76, Arco, etc, it comes from the same regional terminals. You get different additive packages with a few (i.e. "Techron") but the base fuel sent to each station comes from the same pipes.
The difference in quality has more to do with each station. If their equipment maintainenance and processes are up to snuff, and if the employees stick to them under stress, the gas should be fine. If you draw from a tank that was just filled in the last hour (or is still being filled - gak) there's a much higher chance you'll pick up water or other contaminants. If you see a tanker, try the next station. Then are the stations that cut premium or mid-grade to boost profits.
If quality problems were restricted to one or another brand then the choice would be easy, but I've purchased bad gas from every brand over the years. (usually water contamination) and have read about violations from every brand name and independents (R.I.P.).