There is barely a difference, and I would go to the servo in which I get the better service.
FYI, I only go to a local chain for my petrol. They can charge up to three to four cents more per litre, but I don't mind. Why? Because they know me and I have always gotten good service - and sometimes, service is more important than saving a few dollars.
Perhaps things are different in your country, but here... at least in my area, there is no service.
No service at all.
I drive up to a fuel pump kiosk, that accepts a credit card.
Top left of the pump, is where you put in your credit card.
You then fuel your car yourself, and drive away.
You never see any employee of the station, unless you want to buy something from the quick-mart.
So between two gas stations, I'm going to the cheapest one.
Now in places where service matters, then I would agree. Good service is worth a few dollars for sure. If you go to a restaurant, and the service is terrible, saving a few bucks doesn't work. You are going to go to a place that has good service, or you are not going.
But even then, the price still has to be low enough to justify going. Yeah, you can pay your waiters $50,000 a year, and get great quality service. But if the price is too high, no customers are going to show up.
Then all the waiters making $50,000 are unemployed, making zero. There is a trade off, where better service at a higher cost, doesn't work.