Stop in for a cup of coffee

Here is the question.
Have a question that puzzles me. Specifically ignition coils for points ignition systems. When going through my 69 Fury last year I found many things incorrect the way it was setup. I replaced the distributor points and condenser , Cap [i got a few from you]ballast etc to freshen it up. Since it had been running poorly to cap it off might as well get a new ignition coil and keep the old one as a spare. Looking through the application chart for my specific engine ,car etc I saw if I looked carefully there were two different coils offered by different manufacturers.
Specifically a 12 volt and a 6 volt coil. I measured the voltage after the ballast resistor and if I remember correctly was between 7 and 8 volts. Scratching my head I did some research and did find that running a 12 volt coil on 6 or 7 volts would give 50% or more less energy to the plug. I went with the six volt coil and have been running it with no problems,,, So the question in a nutshell,,with a points ,ballast ,ignition system should you run a 6 volt or a 12 volt coil?