I personally think that Butanol would be considered because it is a chemical property that contains four carbon alcohol which is part of why Ethanol was made. I personally think because Butanol is a straight chain primary alcohol to a branched chain tertiary alcohol . I personally think that Butanol is a strong chemical that's why Ethanol had to be combined.
AS of right now Ethanol is the better one, no Butanol. All car engines made from the 1980s and after supports a 10% mixture of ethanol already, and Flexible Fuel Vehicles support an 85% Ethanol blend. Maybe in the future Butanol will become better, but for not are the near future, Ethanol is better.