If you're only charging usb devices then using a 12V battery is extremely inefficient and heavy.
There are plenty of Li ON battery banks out there that can be charged from a USB port and can charge USB devices, you can find them all over amazon. I myself use a box that houses 4 18650 cells of my choosing since my lights can also take 18650 cells, this gives me more options in case any of them go bad and the box can work with less than 4 cells as well. The box has a micro USB input that lets me charge it using my Brunton Explorer solar panel.
I personally wouldn't go with a light that has an internal battery that can't be swapped easily, as the battery is the first thing to go bad on any device.
As to whether you go for Solar or Dynamos that depends on your touring habits and what the weather permits.