davey,
Maybe we're saying the same basic thing. My reasoning for believing that the optical system is superior lies in what I understand to be the way a Hall Effect switch works. The switching occurs at the instant that the ferrite rod begins to LEAVE the field of the coil. The inaccuracy occurs because the distributor components, especially the shaft to bushing and upper shaft to lower shaft clearances cannot be perfect, and that because of wear which has certainly taken place, or will take place. Since the engine runs at different speeds, accellerating and decellerating, That shaft, and therefore the ferrite rods, will move radially relative to the coil, sometimes inward, sometimes outward, and with no great consistancy, sometimes even varying in cycles or sometimes in random patterns. This variation, in a fairly worn system, can actually be pretty significant. An ocilloscope set to raster patern will show this. Since an optical system uses a radially slotted "chopper" disc, this variability is far far less of a factor. This can also be demonstrated with a 'scope. The amount by which this matters in a healthy system doesn't amount to a hill of electrons, but in a worn system, as I stated, it counts quite a bit. But wait, there's more...
This may be the reason I say the optical systems are more accurate, but it's not my major reason for preferring the Crane unit. I LIKE the external amplifier, and I believe it helps starting and low speed spark production and performance. Obviously, I don't have your command of scientific language to explain all this, but that there is what I think. Am I splitting hairs? Perhaps, but I think they are pretty thick hairs. On the other hand, maybe I just like the optical systems because I can more readily identify with them. I used to work with a good friend part time in his precision optical business. I like light!! I've been fascinated with optical sensing units, I admit, since I was a small lad and my dad brought home an "electric eye" for me and him to mess around with. I was dumbstruck fifty years ago, to think that just by shining or not shining a lamp on a thing with wires attatched, I could turn stuff on and off, or make a galvanometer dance around. God, we've come a ways, haven't we? Cheers.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon