The nylon gears are terrific. They last for decades. If something jams the distributor—a dropped screw, say, or a long rotor + ground-off-centre distributor cap (increasingly common due to declining cap/rotor quality) or something else, the gear shears/breaks and the engine stops and that's it.
although I've not heard anything bad about the metal gears
Allow me: if something jams the distributor, the metal gear doesn't shear, then you face the prospect of more stuff breaking below the distributor. AND the engine stops.
careful what you call an upgrade. It may well not be.
