In general there isn't enough difference between the efficiency or drag of most current dynohubs (SON, SP, later Shimano) to be worth bothering about, except that those originally designed for small wheels (SONdelux & SP SD-8) have lower drag and don't give full power until a higher speed compared to those designed for large wheels.
The old shimano DH-3N20 and 3N30 models are also still available but give high-ish lights-off drag and are best suited to town/commuter use.
So, it comes down to cost, weight, guarantee/manufacturer backup and anticipated lifespan.
SON hubs have a 5 year guarantee, and will be refurbished by Schmidt after that. They are very well sealed and many people have used them for many years/miles without problems (one of mine is on its 5th rim), even when abused by thing like riding through deep floodwater. However, they are expensive. Any service or bearing replacement will mean return to the factory in Germany. I believe this is something like £50 plus outward postage after the guarantee has expired.
Shimano hubs use standard Shimano cup & cone bearings, but officially the right-hand bearings are not serviceable because the thin and fragile electrical wiring runs out through them. In practice, it is possible to service them if you are careful. However the bending of the wires required means that even then they are quite likely to break by about the 3rd or 4th service. Reliability is reported to be good, but they are unlikely to withstand floods. The guarantee is 1 year, and you'd probably be unlikely to get anything out of Shimano after that.
SP hubs are too new for any meaningful view of reliability. There is a version in which the generator can be turned off to more or less eliminate lights off drag. They use sealed cartridge bearings which are not user replaceable, and there is as yet no UK service facility, although I believe one is planned. The guarantee is one year, according to Chris Juden at the CTC.
USE/Exposure sell standard SP hubs with their dynamo light. Supernova sell their own version of the SP hubs with upgraded seals and bearings, but this takes the cost up close to that of the SON hubs.
All dynohubs contain strong magnets that cause the axle to try and remain in a number of distinct positions. This doesn't add to the overall wheel drag when riding as each stop has a matching go as the axle snaps forwards to the next position, but it's the cause of many queries when new dynohub users try to twiddle the axle with their fingers.
One consequence of this is that on smooth roads you can sometimes feel a vibration through the handlebars. This will depend on the number and strength of the magnets, your speed, and the resonant frequency of your fork blades. Some people hardly notice, but others do, and dislike it. One who does (
Wouter/swhs) reports that it's most apparent if the SON hubs and least apparent in the SP hubs.
When I used a bottom bracket roller dynamo (Soubitez or Union) I rarely had problems with slip except when it had got packed with dried mud. However, I believe the standard remedy was to roll the roller in a mix of araldite and sand.
