Correction, some ignition coils use external ballast resistors, some have them built in. Depends on what is used on your car. There is no difference in performance other than an external ballast resistor would remain cooler than an internal one. So since a 6 cylinder fires 50% more sparks than a 4 cylinder, there is an advantage in having an external resistor for the 6 cylinder. External resistors require more wiring/labour to install, so they use an internal ballast whenever they can due to lower manufacturing cost.

If you go to electronic ignition, you do not want a ballast resistor at all. (However it will still work with a ballast resistor, it is just not desirable.)