When I put together my fixed gear for winter riding, one of the components I overlooked was lighting. Riding in the winter means riding in the dark, and lighting becomes much more important.
Lighting plays two roles important enough to explicitly enumerate:
One thing I've come to believe is that the greater threat while riding at night comes from the front, and that front-lighting is much more important than rear-lighting. The rationale:
John Forester's pages of lights and the CPSC reflector standards are essential reading.
The result is
State legislatures have codified the importance of a front light; all states require a front light at night, while a rear reflector is sufficient.
Get a front light first.
I ride slowly; in the suburbs and in the city.
The 2.4 watt Lumotec has done pretty well being seen, and it does OK in the gaps between streetlights or under a bridge. It is slightly inadequate on a bike path where there's not good illumination but there's still significant ambient light from the city.
The 6 watt CygoLite did well in the city.
The 10 watt CygoLite spot was great at high-speed narrow paths adjacent to roads with competing headlights.
I'd do off-road riding with pleasure using a 10 watt HID.
Batteries discharge and need to be replaced or recharged.
Generators do not. Hub generators do not slip in the rain or snow, do not tear up the tire or suffer from alignment issues, but introduce continuous drag.
Consider a smart charger. If your battery needs to be on the charger for 18 hours, you can't charge it between getting home and going to work the next day. It's also useful to be able to charge a half-full battery.
As I started commuting in the winter, I realized I could have the light on for three or more hours a day (per charge). An hour in the morning if the weather was inclement; two at night if I went out. The midnight to dawn ride was seven hours of lighting. This kind of riding stretches the capacity of a battery system.
Depending on the battery choice, it may be desirable to deep-discharge the battery. This can greatly complicate planning if your commute can be variable (say, an evening out can add an hour to your ride). Either you're left keeping track of a large number of short trips that take small bites out of the battery, or you're faced with a longer trip at the end of a battery that might exhaust it prematurely.
I did OK with the CygoLite, except for the longer night rides. I also was forced to plan my evenings farther in advance than I'd have liked.
If you're commuting, I suggest lead-acid. Here's why:
SLA is happier with shallow discharge cycles. This means you can charge it daily and life is good. Then you can get a long-lifetime battery which you can discharge deeply if on a long night ride, but you don't have to coordinate your rides to discharge completely. The drawback is that it's heavy.
If you're really interested, this is all explained in Don Klipstein's brilliant article on L.E.D. efficiency. The rest here is my conclusions based on his article.
For colored (red) light, L.E.D. lights are the clear winner. Incandescent lights must be filtered, giving the pure red output of an L.E.D. an enormous advantage over incandescent. Further, they turn on quickly, making them great for use in blinkies, which further extend battery life.
Batteries in blinkies should be replaced more often than most people replace them. I've considered rechargables, but have no experience with them.
For white light, incandescent has been about on par with white L.E.D.s if the power source supplies sufficient voltage. As the voltage falls, current goes up for an incandescent; down for an L.E.D. For my generator-equipped bike, I chose an incandescent for its better beam pattern.
I also have two battery-powered incandescent lights: the Cateye and the CygoLite. The former is a 3 watt light that's quite portable--I carry it when I'm not planning on being out too late, but I've found I often end up cycling home for a bit in the dark despite these plans. It lives in the Camelbak. I also favor incandescent for higher output lighting (6w and above)--again, probably beam pattern, availability, and cost.
If I was planning on regularly running a light on batteries I might have chosen an L.E.D. lamp.
This is an exciting time for white L.E.D.s; efficiency has improved dramatically in the past couple years. I expect there to be improvements in driver electronics to keep the L.E.D.s at their most efficient voltage. The incandescent is not long for this world, despite my choices above.
For long time, I used Trek's Disco Inferno. This taillight with bright L.E.D.s pointing in every direction actually got compliments from drivers when I was stopped at lights. Its weight was too much for its mount, however. The first mount shattered one cold day; Trek kindly sent a replacement. This one broke when I hit a bad pothole. I pinned and expoxied it back together; it broke again in a different place later on. Eventually the light died because of moisture introduced by cracks in the case as a result of its constant falls on the ground.
Buy a good L.E.D. taillight and keep it fed with fresh batteries.
When I went to the generator setup, I ran the wire back to the taillight. This is not a setup I would recommend--the wire is really inelegant, and probably not worth the effort. Still, it provides me with a well-powered taillight that comes on automatically and never needs batteries. Not what I would recommend, but it's what I'd do again.
If it's snowing hard, I add a xenon strobe.
Francis Cooke has a great page of light links.