I have SON and two kinds of Shimano hub dynamos. There are differences but they're too small for someone new to dynamo hubs to notice or, more important, to care about. Unless you're going on a world tour or commuting 10K> miles per year, every year, I don't quite see how the SON price is justified. In days of old, a Shimano hub dynamo was said to be good for 40K, but the more recent Shimanos include a version with Ultegra-level bearings and waterproofing, which must bring it much closer to the SON, which in turn is said to do 60K easily (the 100K you will see thrown about for the SON is kilometres).
However, having said that about the SON price, when you look at the prices of a complete SON wheel and a wheel with one of the better Shimano hub dynamo, the differential narrows, so you might make your evaluation not on the bare dynamo price but on the combo.
Both are rebuildable but the SON only at the factory while Shimano publishes instruction sheets which, in theory at least, permit you to do the job yourself. In practice the Shimano are so cheap that they are not rebuild but replaced. While hub dynamo wheels are rarish on our islands, in Germany they are so common as standard equipment that SON and Shimano hub dynamo wheels are often on manufacturers' surplus sales on German Ebay, and the number of German manufacturers fitting Shimano hub dynamo is definitely on the increase, so it looks like they too think the Shimano is good enough.
Of course, the SON has more street cred down at Le Pub Poseur, but you don't go there, do you...
The rest is magic hidden in the hub.
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