Yes,they are different of course.Wire gauge size relates to the current in the secondary side,not voltage.The heavier windings allow for a stronger magnetic field in the primary,which gives a higher voltage spark on the secondary side.When 12 volt systems came out,the engines were more powerful,turned higher rpm's,so the coils needed to make a more powerful spark.I also believe the secondary windings were changed as well.
A 6 volt coil run on the slightly higher ballast resistor controlled voltage of a 12 volt system won't have a shorter service life,but it will have less voltage and current available to jump the plug gap.
