In my town Shell and Chevon use the same refinery and just use different additives (my uncle worked for shell).
Personally I switch between the two as they use different detergents and figure what one doesn't clean the other will.
Both now meet Top Tier gasoline requirements (it's 4 manufacturers got together and specified minimum requirements of gasoline... If you are wondering I think it was GM, BMW, Honda and toyota)
http://www.toptiergas.com/retailers.html