avoid the pertronix III the tach signal won't work with the OEM tach in the dash if you have one.
the pertronix I works fine.
the pertronix II has a microchip that controls the Dwell on start up to help with cold starting.
the pertronix III has a microchip that controls the dwell on start up and has a rev limiter, and uses a new type of magnetic pickup for the ignition switching signal.
personally i saw no difference between the I and II installed on my car, i currently have a II installed for the last 1.5 years with no issues (knock on wood)
didn't see a difference when i ran points either other then constantly adjusting the dwell.