Warm welcome to you John. I'm not alarmed!
But sorry to say I'm not quite with your mate on that 'donkey / race-horse' analogy though.
A standard so-called 6v-wound dynamo (and field coil) is made of sterner stuff than a finer-wound so-called 12v armature (or field coil ). The sole disadvantage is that the coarser-wound '6v' object needs more revs to get to the point where it can cut in to support a 12v system. Which makes it less good in town riding. But a belt drive with gearing step-up helps here.
A dynamo is delivering all the time loads are applied, whatever the system voltage, so there is no holiday for a dynamo at 6v as opposed to 12.
The '6v' dynamo itself is easily capable of producing the volts (just as '6v' Lucas alternators are too for that matter), so talk of the voltage isn't really much use on its own. A standard E3L is rated at 10A continuous and will deliver its 60 watts till doomsday in a 6v system. At 12v it will safely deliver 80-odd watts continuous, which is under 7A and to that one extent less onerous. A fine-wound so-called '12v' armature running at 12v with a suitably-matched field coil can only safely deliver 60W, ie has a 5A continuous rating. It will cut-in on a 12V system at the same rpm as the coarser-wound item does at 6v, which is why they're quite good things.
I have one bike at std 6v (perfectly OK), another at 12v using a standard 10A dynamo, and one at 12v using fine-wound 5A bits. Both OK too. Living in the country where lots of tickover with the lights on isn't a feature, the standard '6v' dynamo with a solid state regulator at 12v (a DVR2 these days, but a JG when they first came out many years ago) is my favourite and long-proven combination.
To your concern about durability, that dynamo has done 20 years and loads-a-miles (my weakness) with only brushes and bearings being changed. It will support Hothands grips for soft middle-aged hands (32W) plus a 35W halogen light etc, or a 60/55W searchlight otherwise. Which neither of the other options would do as comfortably. None of these dynamos has given a moment's trouble in recent years. I put this down, quite simply, to the excellence of the modern regulators available, which don't allow the field coil to be over-fed (a main cause of armature failures).
As to stripping teeth on belts (the original issue), I can only say that after initial teething (J) troubles with SRM pulleys and tapers a while back, my belt itself has now done 20,000 miles and I haven't given it a second thought. Even better ones may now be available from Dynamo Technologies.
I like dynamos. I prefer them at 12v, but above all I like them with decent solid state regulation - and with a magneto doing the really important thing of guaranteeing locomotion!
