Anywhere you can find a forum post asking whether Tesco petrol is better than BP, you'll see a post by someone who lives near a fuel-depot. They see tankers from all the different companies going in, and decide that all fuel stations must therefore be selling the same fuel.
Because, a fuel depot could never store more than one option could it?
The thing that differs between brands and supermarkets (and within the respective groups) is the additives. These are added, to order, when the tanker is filled and can make a huge difference to the performance of your vehicle. Each vehicle differs, so there's no one correct answer (though we've found Shell generally gives the lowest cost per mile). Engines like to be clean, so the presence of additives can preserve them as well as improve efficiency.