Hi [MENTION=485346]Dornier[/MENTION]. Charging from a dynamo isn't as simple as it sounds. To do it well, it's expensive and probably not worth your while if you don't do stuff like this often. I have a dynamo hub on a couple of my bikes, and I use it to run my lights whenever I ride (day or night) but I don't use it for charging. I usually run a GPS app on my phone when I ride, and to keep the battery from discharging, I use an external battery. I use the Anker Astro E1, which is small, light, and inexpensive. I can take a six hour ride and keep the screen on the whole time, along with a GPS app and a music playing app. At the end of the ride, the external battery is mostly discharged, and my phone battery is still at 100%. If you think this isn't enough, you can bring two batteries.

If you're still interested in dynamos, a hub is best for most people. The drag is a lot lower than the other types, and it's completely silent. The downsides are extra weight and extra cost.