As I understand it, the operating mechanism is different between an original Pertronix Ignitor and the Ignitor II/III. Despite ultimately serving the same purpose and looking outwardly similar, they are different enough that (in my admittedly non-expert opinion) we would do well to be clear which one is under discussion at a given time.
While what Button has said may be true of the traditional Ignitor (that it is basically a magnetic switch that replaces a mechanical one) it is not necessarily true of the Ignitor II/III.
These units have "Adaptive Dwell" and actually vary the dwell time based on sensing the charge level in the coil. As a result, what Spanner Juggler has said above about increased dwell time could sometimes be true (giving a higher resistance coil a longer time to "charge" at low RPM), the converse can also be true: Ignitor II/III can actually reduce the dwell time at low RPM to avoid overheating a lower resistance replacement coil.
A lower resistance coil will charge faster, meaning that it will be able to produce a stronger spark at high RPM when the dwell time is necessarily shorter. As the OP was considering replacing both the points and the coil, this difference between the Ignitor and Ignitor II/III may be worthwhile. (Although perhaps the low-revving TR engine would not benefit much?)
All that being said - for the time being my car still runs points so I cannot comment on the reliability of any electronic ignition. I'm also not an expert and gleaned all of the above from reading as opposed to experience, so take it all with a grain of salt.
