1. USB tail lights: battery can't jiggle loose
2. USB tails lights: not as big of a deal to recharge between every single ride, I keep the bike next to an electrical outlet with a usb charger and charge the light without removing it from the bike
3. Dynamo Tail Lights: the most reliable solution but also the most expensive by far
4. 2 Tail Lights: If one fails the other stays lit