My opinion is you have 2 options. If you want your A to be as close to original as possible "as we do" you keep the 6 volt generator.
If you want to drive your A as a semi modern vehicle, with bright lights, CB's, radios, you use a 12 volt alternator.
Then again, i'm someone who feels a 125A 12volt alternator isn't enough in my cars
An alternator is flat out superior to a DC generator in many ways, from current output, brush wear, voltage regulation and the ONLY reason they used DC generators until the 1960s is they had no way to conveniently rectify high current low voltage AC which an alternator produces.
If they had quality silicon diodes in the 1920s and 30's you can bet your life the model A would've had an alternator.
All that being said, we're keeping a 6V generator on the A because its one of the things that make it what it is, just like the manual timing advance.