Don’t worry about a dynohub’s load-carrying ability: that should never be a problem, even with the extraordinary frontal load of 20kg.
If you want the best, that’ll be Schmidt’s new SON28. On my tourer, I have Shimano’s top model dynamo hub. It’s almost as efficient, reasonably light and durable, but cost less than half as much. The SP hub on the Spa tourer is intermediate in price, equals the SON28 in efficiency and looks very neat, but its closer flanges build a slightly weaker wheel and its small ‘sealed’ bearings are unlikely to last as long, I think, as Shimano’s cup and cone. We won’t really know about that for a year or three.
I use a Busch & Müller E-Werk transformer, but that’s because (in addition to the usual USB devices) I have a 7.6V camera battery. If all you want to charge is a phone and other USB devices, the best lamp will be the Busch & Müller Luxos IQ2-U, which is not only a most excellent headlamp, but also incorporates the electronics to power a USB socket (with a small internal cache battery to keep it powered when you pause), saving the expense and clutter of a separate transformer and its wiring.
This was first published in the June / July 2014 edition of CTC's Cycle magazine.