Hi.
There is no difference between the brands; ex. Shell, Chevron, Mobil. They all meet the minimum federal requirements for unleaded gasoline (if you are fueling up with unleaded gasoline) and even implement what they claim are proprietary additives to make your engine run better. Actually, it doesn't make your engine run better per se, but the compounds used (additives) help keep your engine clean. And they are all very very similar in their molecular structure. It is their structure that gives these compounds their "cleaning" ability, which relies on surface area and non-polarity than anything else. But that would require a lesson in organic chemistry which...haha! Would take a long time to type out.
There are different brands of gasoline, but only one real type of gasoline for passenger vehicles: unleaded. Or probably diesel too. So I don't personally think that there is a discernible difference between the gas brands available for your consumer vehicles. However, fuels developed for racing are a different story, and each is formulated differently.