Usually the better fuel is one that is lower in sulfer. This allows a leaner fuel/air mix without knock, more efficent combustion, lower emmissions and converter life. It can vary by area/refinery supplier as to who is better. But the most refined fuel is usually going to be lower sulfer. In the U.s. at least amoco/bp ultimate supreme is refined an extra step and is generally lower in sulfer. There are also addatives that are different between brands but this realy just makes using an additional fuel system cleaner somewhat less necessary as often.