On a
touring bike? Well, how much
touring (not
commuting, not
randonneuring - there are other, better forums for those fun things to do with a bike) are you going to do in the dark?
I pack a small flashlight - useful for all kinds of touring situations. For the rare twilight ride, I have a
mount.
Excellent advice indeed, for anyone for whom the only battery powered item they take on tour is a light. For others, I submit that a dynamo could be worthwhile during the day as well.
For the OP, hub dynamos seem to be the most widely available, and suitable for the majority of uses. Velological make a lightweight rim dynamo, which has the advantage that you can turn it off (or at least I think you can). You also don't need a wheel rebuild to fit one. Possibly this option may become more widely used in future, but right now the hub type have the ascendency.