If you do get a dynamo, make sure you keep a couple of cheapo blinkies on the bike. No fun at all when the bulbs burn out to be left having to ride home unlit. I'm thinking of the importance of being seen on a wet night, rather than lighting up the road here, but in the end, that is the difference between living and dying. Dynamo bulbs fail too often for my liking, so a fallback is essential.