The advantages of a 12 volt system are that thinner wire can be used and there is less current drop which makes for easier starting. If a 6 volt system is maintained well, however, it worked very well. Dodge may have switched from 12 volts to 6 volts when the Dodge brothers sold out to Chrysler and Dodge became a division of Chrysler.
U.S. cars from the 1930s through 1952 were six volt. General Motors at some point in the 1930s went from positive ground to negative ground. However Ford, Chrysler and I think most of the independents were positive ground.
In 1953, GM introduced the 12 volt negative ground system on the Cadilllac, Oldsmobile, and the Buick Roadmaster and Super models. All the rest of the the GM cars including the Buick Special stayed with the 6 volt system. At the same time the GM cars that used the 12 volts system also introduced air conditioning. In 1954, the Buick Special adopted the 12 volt system. In 1955, all the GM cars switched over to a 12 volt system. The rest of the U.S. manufacturers all switched over to 12 volt negative ground in 1956.
European cars were different. I think that VW didn’t introduce 12 volt system until 1967 (I am not sure about this). I do remember that the Morris Minor line that I saw in1958 had a 12 volt battery. I have no idea about other imports.