Internal resistor coils will have a greater frequency of hard starting problems, especially if the engine has high compression, or other causes of low cranking speed/high starter current. The reason for the 9V coil primary on non-resistor coils is to allow the ignition to work properly during cranking when the battery voltage is lower than normal. If a coil is internally current limited to prevent point burning when operating at the 14+ volt range of normal 12 volt charging systems, the primary current available during cranking at 9 to 10 volts will be low enough to give a very weak spark.
It's simpler to run a low-voltage coil, insert an external resistor during normal running, and bypass the resistor while cranking. HEI ignition systems do not need the resistor, because there are no points to burn during full-voltage running. Use an internal-resistor coil if you choose, but be aware there might be some spark voltage problems during high-load cranking.
Jerry
