Compare the Shimano DH-3N80 (top of Shimano's range) against the Schmidt SON 28 (what they call the standard model).
Electrically, they're the same.
The Shimano (Ultegra-level hub) IIRC has a little more resistance than the Schmidt when the light is on, and a little less when the light is off. The Schmidt is 60g lighter.
It's not possible to service the Schmidt yourself, but you can send it back to Germany to get it done. The Shimano hub is officially non-serviceable. You can it yourself though it's not easy.
The Shimano is (much) less than half the price of the Schmidt.
Just call the drag the the same, on the basis that 20% (or whatever) of hardly anything isn't enough to worry about. It's only worth thinking about on the cheap Shimano hubs (N20, N30)
There are several versions of both the Schmidt and Shimano hubs, and pretty much all the independent comparisons of drag etc refer to the older versions of one or the other hub.
It's the same with weight comparisons - which version?
The Schmidt warranty is 5 years; Shimano will depend on where you bought the hub, but is unlikely to be more than 2.
Other Shimano hubs (eg XT) occasionally have inadequate bearing grease when sold. If this happens with a dynohub, it will be a warranty claim when the hub fails rather than half an hour putting some grease in.
DIY service is possible, but there's a farly high chance of breaking the wiring and rendering the hub useless, even if you've done it OK before.
The Schmidt is better sealed, so you are more likely to get away with something like riding through a flood or crashing in a ford (dynohubs are easily forgotten about during the daytime).
With some bikes there may be no option but Schmidt - there are quite a lot more options.
