THE HUB DYNAMO
Either a Shimano hub dynamo or the SON will be very good. I have both, the Shimano in two grades, one of which has better bearings. Unless you're a major tourer, the SON is huge overkill and I fail to justify its extravagant price every time I use the bike on which I have mine. Good for posing in cafe society though.
The major difference between the Shimano and SON dynamos is that the SON is supposed to see the bike out while the Shimano is built to last 40k klicks. Since a good Shimano dynamo costs less than a third, and sometimes only a quarter, as much as a SON, the decision rests on what sort of brakes you fit, and consequently how often your rims wear through and you rebuild the wheel. If you have rim brakes, there is little reason to splash out on the SON; just buy a new Shimano every time you rebuild the wheel, or every second time; if you fit discs and the rims will therefore possibly last as long as the frame, the SON might eventually justify its cost.
There are some slight differences between the dynamos in how much current they provide at any particular speed. Self-alleged speed kings claim the SON puts out more light at 25mph, below which of course no self-respecting cyclist rides. I'm a downhill racer who slows on the uphills, and I like the output curve of the cheaper Shimano better for providing more light at lower speed.
If you find output curves on the net, be careful on two counts. First, there are some old curves taken from when Shimano took over a Sanyo dynamo design, long outdated; a current Shimano dynamo is in every respect except longevity the equal of the SON. The second problem is that the data, if taken in Germany, is not truly independent because the magazine which published it didn't have the necessary test equipment and therefore used a SON employee and equipment to measure the tests! (I'm sure there was no deliberate distortion, but there needn't be; giving your own guys those slight benefits of the doubt which arise in every test adds up without any volition. The same applies to tests of lamps: Schmidt Maschinenbau, makers of the SON and the Edelux which is a jumped-up Cyo, see below, and BUMM have an incestuous relationship, and guess which engineer was involved in taking the lamp test measurement -- yes, the same Schmidt/SON/Edelux employee...)
By the way, though a good sidewall dynamo is now more expensive than an excellent hub dynamo, if you already have a good sidewall dynamo, there is absolutely no reason to throw it out. It works as well with the lamps recommended below as a hub dynamo, and might offer a weight saving if mass is important to you.
THE REAL QUESTION: THE LAMP YOU BUY
The question isn't actually the dynamo. In my opinion every bike except outright daylight-only racers should have a dynamo, and the common ones are all good. The real question is the lamp that you buy. You need two kinds of lamps. One flashes and makes you visible to drivers; those are normally battery lamps. The other kind throws light and is for you to see the road and obstructions by; there is absolutely no reason today why lamps to see by should not be dynamo lights.
Of lights to see by, powered by a dynamo, anything less than BUMM's Cyo isn't good enough. The lights that aren't good enough include recent halogen and LED lights thought fine until recently. The Cyo, of which the R or nearfield versions are the most useful for speeding around unlit lanes, throws out more light, and better shaped too, than the old 6V Volkswagen Beetle of fond memory. The R version of the Cyo has a useful reflector and a certain amount of (still inadequate -- it's a BUMM lamp after all) sidethrow while the 'racer' Cyo throws no more useful light on the road, nor any further, but lacks the essential facilities of reasonable sidethrow and reflector. The Edelux is an upgraded, more expensive version of the racer's Cyo in flashier dress, including a red that matches the red available on Rohloff hubs; very serious cyclists who have splashed out the extra report that they love the Edelux.
The Cyo has pass-through switching and connections for a LED rear lamp to be dynamo-powered as well.
If the Cyo has a fault it is that the horizon is too well defined, that there isn't enough spill above the horizon to see high-set road signs.
A GROWN-UP LAMP REQUIRES CAUTION
If you do buy a Cyo, be careful about how you set it up; it is strong enough to cause an accident. The law in most countries for dipped lights is something like a metre high horizon at a hundred meters. I used an empty supermarket lot to set up my lights and, in the complex of service roads on the way to the main road, found a car driver slowing to hang out of the window to inspect my lights objectionably closely. I tilted the bike to flash the lights through his eyes and said into his open window, "Park until your eyes recover, or I'll report you to the police." The answer was, "We are the f---ing police!" So I stopped to take their opinion on whether my lights are legal, and learned that this little rule of thumb is a pretty good guide, though my supplementary battery white flasher to the front should preferably be amber.
The rest is magic hidden in the hub.
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